enable dealing with empty metadata in tests

Author: marc-vdm

activity_browser/bwutils/searchengine/base.py

@@ -114,6 +114,9 @@ def update_dict(update_me: dict, new: dict) -> dict:
             update_me = update_me | new_data
             return update_me
 
+        if len(update_df) == 0:
+            return
+
         t = time()
         size_old = len(self.df)
         # identifier to word and df