Verify Edge attributes exist before setting

diagrams/__init__.py

@@ -436,9 +436,12 @@ class Edge:
         for k, v in self._default_edge_attrs.items():
             self._attrs[k] = v
 
+        if label:
             # Graphviz complaining about using label for edges, so replace it with xlabel.
             self._attrs["xlabel"] = label
+        if color:
             self._attrs["color"] = color
+        if style:
             self._attrs["style"] = style
         self._attrs.update(attrs)