[2.x] fix: handle null gotoItem in SearchModal selectResult to prevent cras…

[imorland]

…h on Enter key

When a search source (e.g. GeneralSearchSource in admin) sets data-id on results but returns null from gotoItem(), selectResult() would fall through to item.find('button')[0].click() — crashing when the item contains a link () rather than a button.

Fix by consolidating the link-fallback check to run whenever selectedUrl is still null, and guarding the button click with a length check.

Fixes #4281

**Fixes #4281 **

Changes proposed in this pull request:

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Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)