Return False when any of the operands is invalid.

topydo/lib/Filter.py

@@ -168,6 +168,9 @@ class OrdinalTagFilter(Filter):
 
             if not operand2:
                 operand2 = date_string_to_date(self.value)
+
+            if not operand1 or not operand2:
+                return False
         else:
             try:
                 operand1 = int(p_todo.tag_value(self.key))