(7.3.z) Smallrye OpenAPI throws java.lang.NullPointerException because of null parameter schema

This was spotted while running RESTEasy test suite against EAP XP 1.0.0 CR1 and CR2.
Initially it seemed that the specific test case [1] was failing due to the same root cause of those tracked by the following issue:

• https://issues.redhat.com/browse/JBEAP-19600

Eventually it turned out that the changes to fix the above mentioned issue didn't solve this specific test case failures and a deeper investigation was done, see [2].

Specifically a strange behavior is represented by the fact that the reproducer built manually by using the same resources used by the test case could be deployed successfully while the WAR archive generated by Arquillian in the RESTEasy TS was failing to deploy regularly.

The investigation detailed at https://github.com/smallrye/smallrye-open-api/issues/353#issuecomment-639556113 provided evidence that there are some cases in which the parameter schema would be set to null, so the issue [2] was created upstream.

[1]
https://github.com/resteasy/Resteasy/blob/3.11/testsuite/integration-tests/src/test/java/org/jboss/resteasy/test/form/Resteasy1405Test.java

[2]
https://github.com/smallrye/smallrye-open-api/issues/359