Fix return type of FileGenerator::getClass

[weierophinney]

Provide a narrative description of what you are trying to accomplish:

• [Y ] Is this related to quality assurance?
  Return type of \Zend\Code\Generator\FileGenerator::getClass only has ClassGenerator while it can also return false when the classes array is empty.
  PHPStan will whine about this, because it should always return a true-thy return, also it gives the perception it will always return the ClassGenerator (while it doesn't always do that)

Therefor I added the false to the return type in the docblock.

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Originally posted by @janvernieuwe at zendframework/zend-code#177