### 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 file_is_stdout:
        filetype = provided_file_format or default_format
    elif provided_file_format:
        filetype = provided_file_format
    else:
        filetype = Path(args.file.name).suffix
        filetype = filetype.lower()
        if filetype not in file_formats:
            raise SystemExit(
                f"Unsupported file format. The file must have the extension {', '.join(file_formats)}."
            )
    if args.serialization_format and not filetype == ".env":
        raise SystemExit("Option `--serialization-format` may only be used with file type `env`.")
    with create_session() as session:
        connections = session.scalars(select(Connection).order_by(Connection.conn_id)).all()
    msg = _format_connections(
        conns=connections,
        file_format=filetype,
        serialization_format=args.serialization_format or "uri",
    )
    with args.file as f:
        f.write(msg)
    if file_is_stdout:
        print("\nConnections successfully exported.", file=sys.stderr)
    else:
        print(f"Connections successfully exported to {args.file.name}.")
alternative_conn_specs = ["conn_type", "conn_host", "conn_login", "conn_password", "conn_schema", "conn_port"]
@cli_utils.action_cli
@providers_configuration_loaded
def connections_add(args):