Handle NULL strings gracefully during constant registration

When libcurl is compiled not using e.g. libz or SSL, then a call to
curl_version_info could return NULL in the corresponding fields of
curl_version_info_data.

Passing such NULL pointers down to REGISTER_NS_STRING_CONSTANT results
in a segfault during php startup, so let's check for this special case
and register a NULL constant in this case.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>