Empty yields raise Fatal error

when using empty yields ```yield;``` the parser raises an fatal error:
```Argument 1 passed to PDepend\Source\AST\ASTNode::addChild() must be an instance of PDepend\Source\AST\ASTNode, null given, called in /Users/tschinde/Documents/Github/pdepend/src/main/php/PDepend/Source/Language/PHP/AbstractPHPParser.php on line 6586```

This is a regression test case to prove the failure.