Fix critical memory bug - Fix detecting null params - Added empty array as default param

- Fix a critical memory management bug that occurs when returning a
member from a method that needed local variables, because
ZEPHIR_MM_GROW() was called but ZEPHIR_MM_RESTORE() was not.
- Fixed detecting when null is passed as a parameter in order to assign default value to parameter. It was working when not passing a parameter but not when passing null explicitly.
- Added empty array as possible default parameter to methods. For
instance:
public function mymethod(myParam = [])
- Added getParameter($name) to CommandInterface

Thanks!