Fix undefined variables where files are included

Reviewed the code and ran PHPStan locally. Fixes the errors and ensures that the variables initiated before including the files.

Not entirely relevant to the issue at hand, but it'd be better if we ran the include in a static closure with the minimum variables passed.

Here's an example from PHPStan itself:

            (static function (string $file) use($container) : void {
                require_once $file;
            })($file);

In these cases we could leverage use to pass items by reference.

+1 for RTBC.

Committed f29ceb6 and pushed to 10.0.x. Thanks!

+++ b/core/lib/Drupal/Component/Transliteration/PhpTransliteration.php
@@ -283,12 +283,10 @@ protected function readLanguageOverrides($langcode) {
-    if (!isset($overrides) || !is_array($overrides)) {
-      $overrides = [$langcode => []];
-    }

@@ -311,14 +309,10 @@ protected function readGenericData($bank) {
-    if (!isset($base) || !is_array($base)) {
-      $base = [];
-    }
-
-    // Save this data.

There is a subtle behaviour change here. If the file sets $overrides to something other then an array this would result in it being set. However setting it to something other than an array is an error so it's probably better to fail here.

#3 seems worth a follow-up as it'd further insulate from side-effects.