[JENKINS-72759] fix handling of readonly for select without parent div

[mawinter69]

when no parent div is present that has .jenkins-select we get a null element and the script runs into an error.
This affects all plugins that use <st:adjunct includes="lib.form.select.select"/> but do not have surrounded the select with <div class="jenkins-select">

See JENKINS-72759.

Testing done

interactive testing

Proposed changelog entries

• Restore functionality of some dropdown list form fields outside configuration forms (regression in 2.446).

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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[NotMyFault]

Thanks!

[timja]

any reason to not do this?

Suggested change

	parentDiv != null &&
	parentDiv !== null &&

[mawinter69]

Also it is technically not possible, if parentDiv would be undefined the !== would evaluate to true and the following access to dataset would fail

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!