Fix foreach() warning on non-array value in SiteSettings::applyAvailabilityRecursive() [#3605948]

SiteSettings::applyAvailabilityRecursive() throws a PHP warning when a child element value is false (boolean) instead of an array:
Warning: foreach() argument must be of type array|object, false given in web/modules/contrib/sites/src/Element/SiteSettings.php on line 395

Issue fork sites-3605948

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3605948-fix-foreach-warning changes, plain diff MR !27
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