### Description

Currently, the `airflow variables export` command requires an explicit file path and does not support output to stdout. However connections can be printed to stdout using `airflow connections export -`. This inconsistency between the two export commands can lead to confusion and limits the flexibility of the variables export command.

### Use case/motivation

To bring some consistency, similar to connections, variables should also be printed to STDOUT, using `-` instead of filename. 

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] 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)

args=(
            ARG_DB_REVISION__DOWNGRADE,
            ARG_DB_VERSION__DOWNGRADE,
            ARG_DB_SQL_ONLY,
            ARG_YES,
            ARG_DB_FROM_REVISION,
            ARG_DB_FROM_VERSION,
            ARG_VERBOSE,
        ),
    ),
    ActionCommand(
        name="shell",
        help="Runs a shell to access the database",
        func=lazy_load_command("airflow.cli.commands.db_command.shell"),
        args=(ARG_VERBOSE,),
    ),
    ActionCommand(
        name="check",
        help="Check if the database can be reached",
        func=lazy_load_command("airflow.cli.commands.db_command.check"),
        args=(ARG_VERBOSE, ARG_DB_RETRY, ARG_DB_RETRY_DELAY),
    ),
    ActionCommand(
        name="clean",
        help="Purge old records in metastore tables",
        func=lazy_load_command("airflow.cli.commands.db_command.cleanup_tables"),
        args=(
            ARG_DB_TABLES,
            ARG_DB_DRY_RUN,
            ARG_DB_CLEANUP_TIMESTAMP,