Refactoring logic around evaluating entity

For some reason this was breaking specs once rebased (no shame, it
happens!) Reworking the logic a bit, I did spot one possible bug so
changed the code slightly by adding a SNO (&) and forced a return value
from the extracted method. Specs are now running clean.

lib/rails_erd/domain.rb

@@ -152,8 +152,11 @@ def check_association_validity(association)
     def check_polymorphic_association_validity(association)
       entity_name = association.class_name
       entity = entity_by_name(entity_name)
-      if entity.nil? || entity.generalized?
-        raise "polymorphic interface #{entity_name} does not exist"
+
+      if entity || entity&.generalized?
+        return entity
+      else
+        raise("polymorphic interface #{entity_name} does not exist")
       end
     end