KAFKA-9514; The protocol generator generated useless condition when a field is made nullable and flexible version is used

[dajac]

The protocol generator generates useless conditions when a field of type string is made nullable after the request has been converted to using optional fields.

As an example, we have make the field ProtocolName nullable in the JoinGroupResponse. The JoinGroupResponse supports optional fields since version 6 and the field is nullable since version 7. Under these conditions, the generator generates the following code:

if (protocolName == null) {
 if (_version >= 7) {
   if (_version >= 6) {
     _writable.writeUnsignedVarint(0);
   } else {
     _writable.writeShort((short) -1);
  }
 } else {
   throw new NullPointerException();
 }
}

spotbugs raises an UC_USELESS_CONDITION because _version >= 6 is always true.

This PR fixes the bug by propagating the outer versions to the underlying VersionConditional so it can generate the code accordingly.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[cmccabe]

LGTM. Thanks, @dajac