OWMergeData: Output None for empty tables

Author: janezd

Orange/widgets/data/owmergedata.py

@@ -221,7 +221,8 @@ def dataInfoText(self, data):
 
     def commit(self):
         self.Warning.duplicate_names.clear()
-        if self.data is None or self.extra_data is None:
+        if self.data is None or len(self.data) == 0 or \
+                self.extra_data is None or len(self.extra_data) == 0:
             merged_data = None
         else:
             merged_data = self.merge()