Fix E_DEPRECATED in zend_vm_gen.php

explode(): Passing null to parameter #2 ($string) of type string is
deprecated

Author: MaxSem

Zend/zend_vm_gen.php

@@ -2497,7 +2497,7 @@ function gen_vm($def, $skel) {
             if (ZEND_VM_KIND == ZEND_VM_KIND_GOTO
              || ZEND_VM_KIND == ZEND_VM_KIND_SWITCH
              || (ZEND_VM_KIND == ZEND_VM_KIND_HYBRID && $hot)) {
-                foreach (explode(",", $param) as $p) {
+                foreach (explode(",", $param ?: '') as $p) {
                     $p = trim($p);
                     if ($p !== "") {
                         $params[$p] = 1;