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 pdepend/src/main/php/PDepend/Source/Language/PHP/AbstractPHPParser.php on line 6586
```

This is a regression test case to prove the failure.

I'm not sure how to fix that, so I leave it up to one of the devs here.