fix(php): 修复子类作为父类返回时的类型检查逻辑

- 添加了对 objectClass 是否存在的检查，避免空值导致的错误
- 改进了错误消息，明确显示具体的子类和父类名称
- 确保只有在 objectClass 存在且不等于 returnClass 时才进行抽象类验证

src/Php/CompilerBase.php

@@ -1801,8 +1801,8 @@ class CompilerBase extends \PhpAot\Core\Translator
                 $this->fatalError($v, 'The return type is `' . $returnClass . '`, cannot return an instance of `' . $objectClass . '`');
             }
             // 把子类当做父类返回时，父类必须是抽象类
-            if ($objectClass !== $returnClass and !$this->isAbstractClass($returnClass)) {
-                $this->fatalError($v, 'When returning a subclass instance as its parent type, the parent class must be abstract');
+            if ($objectClass and $objectClass !== $returnClass and !$this->isAbstractClass($returnClass)) {
+                $this->fatalError($v, "When returning a subclass `$objectClass` instance as parent type, the parent class `$returnClass` must be abstract");
             }
         }