cindex.py: Handle NULL pointers when parsing translation units

The code inside cindex.py was comparing NULL pointer returned by
clang_parseTranslationUnit and clang_createTranslationUnit with None.
However, as illustrated by the two tests I've added, those conditions
were ineffective which resulted in assert triggering later on.

Instead, a pointer is now treated as a boolean value, a NULL pointer being
False.

Contributed-by: Xavier Deguillard <deguilx@gmail.com>