bpo-44984: Rewrite test_null_strings in _testcapi (GH-27904)

Test also PyObject_Repr(NULL) and PyObject_Bytes(NULL).
(cherry picked from commit 4d689173861257c69adc6dc1dca4171946ddb57f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>