git_revparse_ext: should return a NULL reference  when the revparse expression doesn't lead to a reference

The following test fails:

```c
void test_refs_revparse__ext_returns_NULL_reference_when_expression_doesnt_lead_to_a_reference(void)
{
	test_object_and_ref(
		"HEAD~3",
		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
		NULL);	// currently returns refs/heads/master
}
```

I believe the same issue arises when having `^` or `:` in the revparse expression.