JENKINS-75593: Use empty string for undefined referencedParameters

[asc3ns10n]

The change in #467 has caused parameters without referenced parameters to no longer render correctly in certain conditions:

1. Reactive Reference Parameter set as Hidden HTML is not being hidden (JENKINS-75593)
2. Filtering for Reactive Parameter doesn't function (JENKINS-75660)

While it seems uncommon to use a Reactive Parameter without references, I rely on Reactive Reference parameters with no references for hidden values or to trigger custom JavaScript.

The undefined errors seem to only occur in pipelines. In freestyle jobs the value is an empty string if there are no referenced parameters. I'm thinking it should default to an empty string in both cases. This would maintain consistency between job types and prevent JS errors when accessing the referencedParameters property.

I can add some tests if this is an acceptable solution.

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[kinow]

Sounds good to me, @asc3ns10n . I will have to check if this was disabled to prevent a bug... but if not we can merge it once the tests are fixed. I can fix them when I have time, or if you can fix them and get a green build here I might be able to quickly review and release soon. Cheers