[BUG] ModelUtils uses wrong variable to log inheritance problem

[cvgaviao]

Description

A wrong variable are being used to log, then a null value are written in the console:

DEBUG [2019-12-04 12:00:11,720] [main] Not a parent since discriminator.propertyName is not set null
WARN  [2019-12-04 12:04:01,883] [main] [deprecated] inheritance without use of 'discriminator.propertyName' is deprecated and will be removed in a future release. Generating model for null

This way it is impossible to identify which schema is not fulfilling the inheritance requirements.

openapi-generator version

4.2.3-SNAPSHOT

Suggest a fix

ModelUtils line 930 and 944.

There are a dependency of this error with the inheritance being treated with this: #4454