### 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)

db_create_default_connections()
def _format_connections(conns: list[Connection], file_format: str, serialization_format: str) -> str:
    if serialization_format == "json":
        serializer_func = lambda x: json.dumps(_connection_to_dict(x))
    elif serialization_format == "uri":
        serializer_func = Connection.get_uri
    else:
        raise SystemExit(f"Received unexpected value for `--serialization-format`: {serialization_format!r}")
    if file_format == ".env":
        connections_env = ""
        for conn in conns:
            connections_env += f"{conn.conn_id}={serializer_func(conn)}\n"
        return connections_env
    connections_dict = {}
    for conn in conns:
        connections_dict[conn.conn_id] = _connection_to_dict(conn)
    if file_format == ".yaml":
        return yaml.dump(connections_dict)
    if file_format == ".json":
        return json.dumps(connections_dict, indent=2)
    return json.dumps(connections_dict)
def _is_stdout(fileio: io.TextIOWrapper) -> bool: