[libc] fix behavior of strrchr(x, '\0') (#112620)
`strrchr("foo", '\0')` is defined to point to the end of `foo`, rather
than returning NULL. This wasn't caught by tests, since llvm-libc's
`ASSERT_STREQ(nullptr, "");` is not an assertion error.
While I'm here, refactor the test slightly to check for NULL more
specifically. I considered adding fancier `ASSERT`s (and changing the
semantics of `ASSERT_STREQ`), but opted for a more local fix by fair
dice roll.
