Enh: suppress "Empty Query" results for semantic_search()

[Masltov]

If the provided mask doesn't match any entity in semantic_search(), a "Query returned no results" is created.
Why the print shall be omited in my view:

• The library is used programmatically, hence no user interaction takes place without an overlay --> print is worthless (besides debug)
• Empty results can be detected by checking against an empty list

[Masltov]

Suggestion:
Change optional parameter debug of class SearchParameter in wiki_tools.py to False by default