[TypeDeclaration] Extend ReturnTypeDeclarationRector with incorrect types override

[TomasVotruba]

class SomeClass
{
    private $decodedContents;

    public function __construct(?array $decodedContents)
    {
        $this->decodedContents = $decodedContents;
    }


-    public function getDecodedContents(): array
+    public function getDecodedContents(): ?array
    {
        return $this->decodedContents;
    }
}