### Apache Airflow version

2.2.0 (latest released)

### Operating System

Ubuntu 20.04.3 LTS

### Versions of Apache Airflow Providers

apache-airflow-providers-amazon==2.3.0
apache-airflow-providers-celery==2.1.0
apache-airflow-providers-cncf-kubernetes==2.0.3
apache-airflow-providers-docker==2.2.0
apache-airflow-providers-elasticsearch==2.0.3
apache-airflow-providers-ftp==2.0.1
apache-airflow-providers-google==6.0.0
apache-airflow-providers-grpc==2.0.1
apache-airflow-providers-hashicorp==2.1.1
apache-airflow-providers-http==2.0.1
apache-airflow-providers-imap==2.0.1
apache-airflow-providers-microsoft-azure==3.2.0
apache-airflow-providers-mysql==2.1.1
apache-airflow-providers-odbc==2.0.1
apache-airflow-providers-postgres==2.3.0
apache-airflow-providers-redis==2.0.1
apache-airflow-providers-sendgrid==2.0.1
apache-airflow-providers-sftp==2.1.1
apache-airflow-providers-slack==4.1.0
apache-airflow-providers-sqlite==2.0.1
apache-airflow-providers-ssh==2.2.0


### Deployment

Other Docker-based deployment

### Deployment details

Airflow is deployed through Kubernetes and configured via environment variables.

### What happened

Enabling airflow sentry via these env vars:
```
AIRFLOW__SENTRY__SENTRY_ON="True"
SENTRY_DSN=<dsn>
SENTRY_ENVIRONMENT=<env>
```
Worked fine in airflow 2.1.4.

Upgrading to Airflow 2.2 and staring up any container fails with warning and then an exception "airflow.exceptions.AirflowConfigException: section/key [sentry/before_send] not found in config"

Adding the following env var fixes it:
```
AIRFLOW__SENTRY__BEFORE_SEND=""
```


### What you expected to happen

The new env var should not be required and should have a reasonable fallback.
I believe the code change in #18261 should have had a `fallback=None` arg passed in to the `config.getimport`call like I see in other calls to `getimport`


### How to reproduce

Set up env vars

```
AIRFLOW__SENTRY__SENTRY_ON="True"
SENTRY_DSN=<dsn>
SENTRY_ENVIRONMENT=<env>
```

Run any airflow command.

Tail the logs
```
WARNING - section/key [sentry/before_send] not found in config
Traceback (most recent call last):
  File "/home/airflow/.local/bin/airflow", line 8, in <module>
    sys.exit(main())
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/__main__.py", line 40, in main
    args.func(args)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/cli/cli_parser.py", line 47, in command
    func = import_string(import_path)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/module_loading.py", line 32, in import_string
    module = import_module(module_path)
  File "/usr/local/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 843, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/cli/commands/db_command.py", line 24, in <module>
    from airflow.utils import cli as cli_utils, db
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/utils/db.py", line 27, in <module>
    from airflow.jobs.base_job import BaseJob  # noqa: F401
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/__init__.py", line 19, in <module>
    import airflow.jobs.backfill_job
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/jobs/backfill_job.py", line 28, in <module>
    from airflow import models
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/__init__.py", line 20, in <module>
    from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/baseoperator.py", line 61, in <module>
    from airflow.models.taskinstance import Context, TaskInstance, clear_task_instances
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/models/taskinstance.py", line 84, in <module>
    from airflow.sentry import Sentry
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/sentry.py", line 190, in <module>
    Sentry = ConfiguredSentry()
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/sentry.py", line 112, in __init__
    sentry_config_opts['before_send'] = conf.getimport('sentry', 'before_send')
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/configuration.py", line 477, in getimport
    full_qualified_path = conf.get(section=section, key=key, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/configuration.py", line 373, in get
    return self._get_option_from_default_config(section, key, **kwargs)
  File "/home/airflow/.local/lib/python3.8/site-packages/airflow/configuration.py", line 383, in _get_option_from_default_config
    raise AirflowConfigException(f"section/key [{section}/{key}] not found in config")
airflow.exceptions.AirflowConfigException: section/key [sentry/before_send] not found in config
```

### Anything else

This is only hit by users that enable sentry, as in the default airflow configuration this code is not executed.

### Are you willing to submit PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/sentry.py
#
#
#
"""Sentry Integration"""
import logging
from functools import wraps
from airflow.configuration import conf
from airflow.utils.session import find_session_idx, provide_session
from airflow.utils.state import State
log = logging.getLogger(__name__)
class DummySentry:
    """Blank class for Sentry."""
    @classmethod
    def add_tagging(cls, task_instance):
        """Blank function for tagging."""
    @classmethod
    def add_breadcrumbs(cls, task_instance, session=None):
        """Blank function for breadcrumbs."""
    @classmethod
    def enrich_errors(cls, run):