Add default value for TypePart::$typeNullable to prevent a fatal error

…when setNullable has not been called explicitly.

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

@@ -19,7 +19,7 @@ trait TypePart {
 	private $typeDescription;
 
 	/** @var bool */
-	private $typeNullable;
+	private $typeNullable = false;
 
 	/**
 	 * Sets the type