Suppress -Wformat-overflow warnings in 9.2's xml.c.

Suppress -Wformat-overflow warnings in 9.2's xml.c.

If not using --with-libxml, late-model gcc complains about "'%s'
directive argument is null" in places where xml.c passes the result
of map_sql_identifier_to_xml_name() to %s.  That's not so surprising,
because without USE_LIBXML the body of that function is

	NO_XML_SUPPORT();
	return NULL;

and before 9.3 the compiler won't understand that NO_XML_SUPPORT()
doesn't return.  Silence the warnings by returning "" instead.
We don't need this hack in later branches, AFAICT.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com