Fix the Tag.is_empty() method

The `Tag.is_empty()` method was returning the inverse of the
expected value.

Fixes #12579.

gramps/gen/lib/tag.py

@@ -133,7 +133,7 @@ def is_empty(self):
         :returns: True if the Tag is empty
         :rtype: bool
         """
-        return self.__name != ""
+        return not self.__name
 
     def are_equal(self, other):
         """