Resolve "White screen of death on creating new project"

What does this MR do and why?

When disabling/unchecking all imported sources under Admin > General > Visibility & Access controls, the New project page (projects/new) fails loading and throws a JS error as some input elements are not available on the page and thus event listeners cannot be added.

This regression was most likely introduced in https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/projects/project_new.js#L279 when we migrated jQuery selectors to native JS selectors.

This MR adds a sanity check for the relevant inputs.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #366308 (closed)