### 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)

"""Helps export all the variables to the file."""
    var_dict = {}
    with create_session() as session:
        qry = session.scalars(select(Variable))
        data = json.JSONDecoder()
        for var in qry:
            try:
                val = data.decode(var.val)
            except Exception:
                val = var.val
            var_dict[var.key] = val
    with open(filepath, "w") as varfile:
        varfile.write(json.dumps(var_dict, sort_keys=True, indent=4))
    print(f"{len(var_dict)} variables successfully exported to {filepath}")