pytype: add assertion to guide pytype through Whoosh usage

Mute pytype warnings:

File "kallithea/controllers/admin/admin.py", line 105, in _journal_filter: No attribute 'fieldname' on None [attribute-error]
  In Optional[Any]
File "kallithea/controllers/admin/admin.py", line 106, in _journal_filter: No attribute 'text' on None [attribute-error]
  In Optional[Any]
File "kallithea/controllers/admin/admin.py", line 107, in _journal_filter: No attribute 'startdate' on None [attribute-error]
  In Optional[Any]
File "kallithea/controllers/admin/admin.py", line 107, in _journal_filter: No attribute 'enddate' on None [attribute-error]
  In Optional[Any]

kallithea/controllers/admin/admin.py

@@ -102,6 +102,7 b' def _journal_filter(user_log, search_ter'
         if not isinstance(qry, query.And):
             qry = [qry]
         for term in qry:
+            assert term is not None, term
             field = term.fieldname
             val = (term.text if not isinstance(term, query.DateRange)
                    else [term.startdate, term.enddate])