Better handle empty results from tantivy (#155)

Closes #154

---------

Co-authored-by: Chang She <chang@lancedb.com>

python/lancedb/fts.py

@@ -118,6 +118,8 @@ def search_index(
     query = index.parse_query(query)
     # get top results
     results = searcher.search(query, limit)
+    if results.count == 0:
+        return tuple(), tuple()
     return tuple(
         zip(
             *[

python/lancedb/query.py

@@ -164,6 +164,8 @@ class LanceFtsQueryBuilder(LanceQueryBuilder):
         index = tantivy.Index.open(index_path)
         # get the scores and doc ids
         row_ids, scores = search_index(index, self._query, self._limit)
+        if len(row_ids) == 0:
+            return pd.DataFrame()
         scores = pa.array(scores)
         output_tbl = self._table.to_lance().take(row_ids, columns=self._columns)
         output_tbl = output_tbl.append_column("score", scores)