Don't call Py_DECREF on null in _ttconv.cpp

Clang's static checker noted that if PyBytes_FromString returns NULL
(presumably rare) then Py_DECREF gets called on NULL; throw an
exception instead. Also assert that the char* inputs are not NULL,
since the Python API functions don't usually check that.
Merge pull request #2989 from jkseppan/tidyup-ttconv

Don't call Py_DECREF on null in _ttconv.cpp