Fixed bug that made metal-metal vias think they were well contacts

Author: SWalker

compiler/base/contact.py

@@ -41,7 +41,7 @@ def __init__(self, layer_stack, dimensions=(1, 1), directions=None,
             self.add_comment("implant type: {}\n".format(implant_type))
             self.add_comment("well_type: {}\n".format(well_type))
 
-        self.is_well_contact = implant_type == well_type
+        self.is_well_contact = (implant_type == well_type) and implant_type is not None
 
         # If we have a special tap layer, use it
         self.layer_stack = layer_stack