Use nullable return type for TypePart::getTypeNullable()

This is to prevent a fatal error when setNullable has not been called explicitly,
when generateNullableTypes is enabled.

Error was:
  PHP Fatal error:  Uncaught TypeError: Return value of
  gossi\codegen\model\PhpParameter::getNullable() must be
  of the type bool, null returned in
  […]/vendor/gossi/php-code-generator/src/model/parts/TypePart.php:75

Author: bnf

src/model/parts/TypePart.php

@@ -71,7 +71,7 @@ public function getTypeDescription(): ?string {
 	 * 
 	 * @return bool
 	 */
-	public function getNullable(): bool {
+	public function getNullable(): ?bool {
 		return $this->typeNullable;
 	}