Extend tests to call ASN1_{GENERALIZED,UTC,}TIME_set_string() with a NULL

Extend tests to call ASN1_{GENERALIZED,UTC,}TIME_set_string() with a NULL
pointer - because, you know, you might want to set a string on a NULL
object. The previous implementation apparently allowed this as a way of
testing if the string was valid... probably because the *_check() functions
are only useable after the string has already been set.