Return false for undefined functions, not null

Calling an undefined function like `{{ foo('bar') }}` results in a fatal error being thrown:

    Fatal error: Call to a member function getSafe() on a non-object in
    .../vendor/twig/twig/lib/Twig/NodeVisitor/SafeAnalysis.php on line 84

According to the Twig docs for [registerUndefinedFunctionCallback()](http://twig.sensiolabs.org/doc/recipes.html#defining-undefined-functions-and-filters-on-the-fly), `false` should be returned instead of `null`, if the requested function is undefined.

This fix changes the return value so the correct error message is thrown:

    The function "foo" does not exist in "..." at line 4