Fixes T97794 (which is a reintroduction of an older issue T67266 that has been fixed in the python importer, but the fix was not in the C++ one). Some software produces OBJ files with mtllib statements like mtllib "file name in quotes.mtl", and the new importer was not stripping the quotes away.

While at it, I noticed that MTLParser constructor was taking a StringRef and treating it as a zero-terminated string, which is not necessarily the case. Fixed that by explicitly using a StringRefNull type.