For consistency with imagegd(), imagegd2() and imagexbm(), which throw
a TypeError if a path containing NUL bytes is given, we throw a
TypeError for the other image writer functions as well.
These tests actually check that no file with a name containing a NUL is
created by the GD image output functions. This is superfluous, since
it is sufficient to check that the function failed, and that an
appropriate warning has been raised.
We also add missing nullbyte injection tests.
