fix on retrieval when key_term length is zero

The following error is solved:

    if core_text[x].lower() == key_term[0].lower():
IndexError: string index out of range

llmware/retrieval.py

@@ -1374,6 +1374,9 @@ def locate_query_match (self, query, core_text):
         for x in range(0, len(core_text)):
             match = 0
             for key_term in query_tokens:
+                if len(key_term) == 0:
+                    continue
+
                 if key_term.startswith('"'):
                     key_term = key_term[1:-1]