Improve the return type of ParameterBag::getEnum

The return value can only be null when using null as default value. when
providing an enum instance as default value, we know that the return
value will always be an enum instance.

Author: stof

InputBag.php

@@ -83,6 +83,8 @@ public function set(string $key, mixed $value): void
      * @param ?T              $default
      *
      * @return ?T
+     *
+     * @psalm-return ($default is null ? T|null : T)
      */
     public function getEnum(string $key, string $class, ?\BackedEnum $default = null): ?\BackedEnum
     {

ParameterBag.php

@@ -160,6 +160,8 @@ public function getBoolean(string $key, bool $default = false): bool
      * @param ?T              $default
      *
      * @return ?T
+     *
+     * @psalm-return ($default is null ? T|null : T)
      */
     public function getEnum(string $key, string $class, ?\BackedEnum $default = null): ?\BackedEnum
     {