Changed scalar $value type-hint to scalar|null

Co-authored-by: Paweł Niedzielski <pawel.niedzielski@ibexa.co>

src/contracts/Output/Generator.php

@@ -197,7 +197,7 @@ public function valueElement(string $name, $value): void
     }
 
     /**
-     * @phpstan-param scalar $value
+     * @phpstan-param scalar|null $value
      * @phpstan-param array<string, scalar> $attributes
      */
     abstract public function startValueElement(string $name, $value, array $attributes = []): void;