[bindings] allow null strings in Python 3

Some API calls accept 'NULL' instead of a char array (e.g. the second
argument to 'clang_ParseTranslationUnit').  For Python 3 compatibility,
all strings are passed through 'c_interop_string' which expects to
receive only 'bytes' or 'str' objects.  This change extends this
behavior to additionally allow 'None' to be supplied.

A test case was added which breaks in Python 3, and is fixed by this
change.  All the test cases pass in both, Python 2 and Python 3.

llvm-svn: 316264