Fix TypeError in switchActiveLanguage on pages without primary LSB

[GasparSukk]

Summary

switchActiveLanguage() throws TypeError: undefined is not an object (evaluating 'a.length') on admin pages that do not render the primary LSB meta-box — ACF options pages are the most common case, but any page that only renders per-field/editor LSBs (no qtranxs-meta-box-lsb) is affected.

Root cause

js/core/hooks/handlers.js line 888 and 900:

const tabSwitches = _tabSwitchElements[_activeLanguage];
for (let i = 0; i < tabSwitches.length; ++i) { ... }

_tabSwitchElements is populated only by createSetOfLSB() (the primary meta-box LSB). On pages that don't use it, _tabSwitchElements[_activeLanguage] is undefined and .length throws. The crash aborts the language switch, so nothing updates.

This is a regression introduced in 3.16.0 via the JS hooks refactor (#1464 / #1469). 3.15.x did not touch _tabSwitchElements inside switchActiveLanguage.

Fix

Guard both reads with || [] so the loop no-ops when there are no primary LSBs to update. Per-field/editor LSBs and the rest of the switchActiveLanguage flow (wp.hooks actions, _onLoadLanguage, etc.) continue normally.

-        const tabSwitches = _tabSwitchElements[_activeLanguage];
+        const tabSwitches = _tabSwitchElements[_activeLanguage] || [];

Applied at both call sites in switchActiveLanguage.

Test plan

• Reproduce on 3.16.1: open an ACF options page, click any per-field language tab → TypeError in console.
• With the patch applied: language switches work across all fields on the same page.
• Regression: post-edit screen (which DOES have the primary LSB meta-box) still toggles the primary LSB active class correctly, because the key is populated normally.