Fix: Issue with typing null monomial

mapytex/calculus/core/typing/multiply.py

@@ -34,6 +34,17 @@ def multiply_filter(left, right):
     :param right: MO
     :returns: MO if it is a special case, nothing other wise
     """
+    try:
+        if left == 0:
+            return left
+    except TypeError:
+        pass
+    try:
+        if right == 0:
+            return right
+    except TypeError:
+        pass
+
     try:
         if left == 1:
             return right