### 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)

if not os.path.isdir(arg):
            parser.error(f"The directory '{arg}' does not exist!")
        return arg
def positive_int(*, allow_zero):
    """Define a positive int type for an argument."""
    def _check(value):
        try:
            value = int(value)
            if allow_zero and value == 0:
                return value
            if value > 0:
                return value
        except ValueError:
            pass
        raise argparse.ArgumentTypeError(f"invalid positive int value: '{value}'")
    return _check
def string_list_type(val):