[BUGFIX] Guard version switcher against a missing selector

[CybotTM]

Problem

On the search results page (/search) the version-switcher script throws uncaught errors, which abort the remaining page scripts:

Uncaught TypeError: Cannot read properties of null (reading 'getAttribute')
Uncaught TypeError: Cannot read properties of null (reading 'addEventListener')

versions.js looks up #versionSelect / #languageSelect and then calls versionSelect.getAttribute(...) unconditionally. Those elements only exist on manual pages — on the search results page versionSelect is null.

See it live (current production, not yet fixed — open the browser console): https://docs.typo3.org/search/search?q=openai&filters%5Bpackage%5D=netresearch/nr-llm

Fix

Bail out early in versions.js when the switcher is absent. Adds a regression test and rebuilds theme.min.js (grunt js).

Verification — console before / after

Headless Chromium against the live page above, routing theme.min.js to the build from this PR:

BEFORE (production theme.min.js)
  - Unexpected token 'export'                                    <- typo3infrastructure/universe (separate, internal)
  - Cannot read properties of null (reading 'addEventListener')  <- versions.js
  - Cannot read properties of null (reading 'getAttribute')      <- versions.js

AFTER (this PR's theme.min.js)
  - Unexpected token 'export'                                    <- unchanged (universe; out of scope here)

Both versions.js null errors are gone; only the unrelated universe error remains. npm test -> 28 passed.

Note

The Unexpected token 'export' comes from the typo3infrastructure/universe bundle (outside this repository) — a separate issue for the infrastructure maintainers, not addressed here.