Avoid defaulting the final $flags argument to null

The correct default for the $flags argument is 0, not null. Passing a
null causes a warning.

Author: kovshenin

agent/php_mysqli.c

@@ -434,7 +434,13 @@ static nr_status_t nr_php_mysqli_link_real_connect(
     ADD_IF_STR_SET(argv, argc, metadata->database);
     ADD_IF_INT_SET(argv, argc, metadata->port);
     ADD_IF_STR_SET(argv, argc, metadata->socket);
-    ADD_IF_INT_SET(argv, argc, metadata->flags);
+
+    /*
+     * Avoid defaulting the final $flags argument to null.
+     */
+    if (metadata->flags) {
+      ADD_IF_INT_SET(argv, argc, metadata->flags);
+    }
   }
 
   retval = nr_php_call_user_func(link, "real_connect", argc, argv TSRMLS_CC);