Is printing a null-pointer Undefined Behavior?

Question

When studying the sample code for this question I had assumed it was Undefined Behaviour which was preventing subsequent uses of std::cout from printing. But it

Answer 1

gcc ostream.tcc line 319:

template
  basic_ostream<_CharT, _Traits>&
  operator<<(basic_ostream<_CharT, _Traits>& __out, const char* __s)
  {
    if (!__s)
__out.setstate(ios_base::badbit);

gcc is simply performing a check that the standard does not guarantee, which is fine because it's undefined anyway.