Use nullable return type for TypePart::getTypeNullable()

[bnf]

…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

Stack trace:
#0 […]/vendor/gossi/php-code-generator/src/generator/builder/parts/TypeBuilderPart.php(53): gossi\codegen\model\PhpParameter->getNullable()
#1 […]/vendor/gossi/php-code-generator/src/generator/builder/ParameterBuilder.php(16): gossi\codegen\generator\builder\ParameterBuilder->getType(Object(gossi\codegen\model\PhpParameter), true, true)
#2 […]/vendor/gossi/php-code-generator/src/generator/builder/AbstractBuilder.php(41): gossi\codegen\generator\builder\ParameterBuilder->build(Object(gossi\codegen\model\PhpParameter))
#3 […]/vendor/gossi/php-code-generator/src/generator/builder/parts/RoutineBuilderPart.php(40): gossi\codegen\generator\builder\AbstractBuilder->generate(Object(gossi\codegen\model\PhpParameter))
#4  in […]/vendor/gossi/php-code-generator/src/model/parts/TypePart.php on line 75

[bnf]

Hi, I forgot to add a test for the initial pull request. Sorry for that – I've just added one. Hope you'll find some free time to review this one. Thank you in advance.

Looking forward to a 0.6 release containing this fix :)