[bindings] allow null strings in Python 3

[frutiger]

Summary

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.

[frutiger]

Friendly poke!

[frutiger]

Landed in rL316264.

bindings/python/clang/cindex.py

                 return cls(param)
             if isinstance(param, bytes):
                 return cls(param)
+            if param is None:
+                # Support passing null to C functions expecting char arrays
+                return None
             raise TypeError("Cannot convert '{}' to '{}'".format(type(param).__name__, cls.__name__))
 
         @staticmethod

bindings/python/tests/cindex/test_index.py

     assert isinstance(index, Index)
     tu = index.parse(os.path.join(kInputsDir, 'hello.cpp'))
     assert isinstance(tu, TranslationUnit)
+    tu = index.parse(None, ['-c', os.path.join(kInputsDir, 'hello.cpp')])
+    assert isinstance(tu, TranslationUnit)