Fix #18715 [JAVA] ensure codegen fields and additionalProperties are not conflicting

[jpfinne]

Only write back to additionalProperties if it was present

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[wing328]

can you please resolve the merge conflicts when you've time? then I'll get it merged first thing before merging other PRs

[jpfinne]

can you please resolve the merge conflicts when you've time? then I'll get it merged first thing before merging other PRs

Done

[wing328]

cc @OpenAPITools/generator-core-team

[wing328]

we can merge it to start with. i've a few more qeustions/suggestions that I'll discuss with you via Slack