Fix: an existing index == None should return False

While using `migra`, I got the following error: `AttributeError: 'NoneType' object has no attribute 'name'`.  Using `pdb`, I determined that the error arose when a table with an index was compared to one without an index.  This patch eliminated the error.

Author: SKalt

schemainspect/pg/obj.py

@@ -436,9 +436,12 @@ def create_statement(self):
 
     def __eq__(self, other):
         """
-        :type other: InspectedIndex
+        :type other: Optional[InspectedIndex]
         :rtype: bool
         """
+        if other is None:  # sometimes the other index doesn't exist
+            return False
+        
         equalities = (
             self.name == other.name,
             self.schema == other.schema,