stdlib.api: Use a fallback logger
stdlib.api: Use a fallback logger

`leapp.libraries.stdlib.api.current_logger` currently fails if no actor
is currently running. However this function is used by some other
libraries that, at least in test cases need to do the logging too.

This change introduces a fallback in case current_actor returns None.

Signed-off-by: Vinzenz Feenstra <600e8a10eeff4f08c65e2cf493cd851025e86fd2@redhat.com>

leapp-to/prototype,leapp-to/prototype,leapp-to/prototype,leapp-to/prototype