NIFI-16276 fix: Updating process group version should not clear parameter context

[esecules]

Summary

NIFI-16276

Under certain circumstances an instance of a versioned process group could have its parameter context overwritten to null if the proposed new version of the process group doesn't have a parameter context. The fix is simple. Just remove the leading if block that sets the parameter context to null when the proposed version has no parameter context but the current version does.

The rationale is that users might make versions of common components which they version without a specific parameter context and then when they place instances of them on the canvas, they will assign a parameter context to that instance, but on every version upgrade that parameter context gets set back to null.

There is a workaround to the problem that might not be obvious. Versioning a process group with an empty parameter context is enough, but there's no easy path to push users in that direction either.

It is worth noting that thins change would alter the behaviour when it comes to publishing a new version of a process group where you remove the parameter context from version control. No longer would instances of a process group also null out their parameter context.

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