Add a comment explaining why we return

Thanks @leplatrem for the suggestion.

Author: glasserc

autograph_utils/__init__.py

@@ -422,6 +422,8 @@ def _check_name_constraints(self, current_cert, next_cert):
                 cryptography.x509.NameConstraints
             ).value
         except x509.ExtensionNotFound:
+            # No name constraints. This cert is therefore OK to sign
+            # any name whatsoever.
             return
 
         name = next_cert.subject.get_attributes_for_oid(NameOID.COMMON_NAME)[0].value