Merge branch '7.3' into 7.4

* 7.3:
  use the empty string instead of null as an array offset
  pass the empty string instead of null as key to array_key_exists()

Author: xabbuh

ChoiceList/ArrayChoiceList.php

@@ -105,7 +105,7 @@ public function getChoicesForValues(array $values): array
         $choices = [];
 
         foreach ($values as $i => $givenValue) {
-            if (\array_key_exists($givenValue, $this->choices)) {
+            if (\array_key_exists($givenValue ?? '', $this->choices)) {
                 $choices[$i] = $this->choices[$givenValue];
             }
         }