Allow mistmatched joins with an empty table

src/kbmod/results.py

@@ -180,7 +180,7 @@ def extend(self, results2):
         ------
         Raises a ValueError if the columns of the results do not match.
         """
-        if set(self.colnames) != set(results2.colnames):
+        if len(self) > 0 and set(self.colnames) != set(results2.colnames):
             raise ValueError("Column mismatch when merging results")
 
         self.table = vstack([self.table, results2.table])