[4.x] Fix select fieldtype fallback differently

[jasonvarga]

This is an alternate solution of #9038.
The issue was that Arr::get will return only return the fallback if the key is completely missing.
In the bug's case, the key (the value) was there but the value (the label) was null.
This adjusts it to use a ?? so we always get the fallback and never null.

$arr = [
  'alfa' => 'Alfa',
  'bravo' => null,
];

Arr::get($arr, 'alfa', 'fallback'); // "Alfa"
Arr::get($arr, 'bravo', 'fallback'); // null - We were expecting "fallback"
Arr::get($arr, 'charlie', 'fallback'); // "fallback"