db2_last_insert_id() may return null

[phansys]

See:

pecl-database-ibm_db2/ibm_db2.c

Lines 7491 to 7495 in 255cfca

	if (last_id[0] != '\0') {
	ZEND_RETURN_STRING(last_id, 0);
	} else {
	RETURN_NULL();
	}