fix: make wizard prompt code clearer and fix crash
- docstring for get_response_from_menu_prompt() now documents all the
parameters, and all the possible return types based on the parameters.
- fix: get_response_from_menu_prompt() no longer ever returns None: when there
are no selections with multi=True, you can an empty tuple, so you can always
iterate over the return value of this function if you said multi=True.
This fixes a bug in the wizard where you'd get an exception if you selected
no text processing steps.
- Refactor get_response_from_menu_prompt() to make its logic clearer.
- Remove redundant multi=False, search=False and return_indices=False in all
the places where we call get_response_from_menu_prompt(), since they don't
really make the code any clearer. We only need to specify them when we
deviate from the defaults.
- Augment unit testing to exercise None as simple_term_menu return value.
