impr(settings): prevent customLayoutFluid and customPolyglot to be empty (@fehmer)

[fehmer]

Prevent user from deselecting all layouts/languages in customLayoutfluid and customPolyglot selects.

Fixes FRONTEND-MW and FRONTEND-MS

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: frontend/src/ts/pages/settings.ts

Function	Unhandled Issue
fillSettingsPage	Error: Failed to find a button element for fontFamily /set... Event Count: 24 Affected Users: 79
fillSettingsPage	TypeError: Cannot set properties of null (setting 'innerHTML') ... Event Count: 10 Affected Users: 107

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[Miodec]

Thanks