fix(generator): 修复只读属性的默认值初始化问题

- 在生成 PHP stub 时为只读属性使用 ZVAL_UNDEF 初始化
- 防止对只读属性进行类型默认值初始化
- 添加空指针检查以避免访问可能不存在的变量名称

src/Php/Analysis/SsaBuilder.php

@@ -984,7 +984,7 @@ class SsaBuilder
                 $defs[] = $stmt->valueVar->name;
             }
         } elseif ($stmt instanceof Stmt\Catch_) {
-            if (is_string($stmt->var->name)) {
+            if ($stmt->var && is_string($stmt->var->name)) {
                 $defs[] = $stmt->var->name;
             }
         }
@@ -1223,7 +1223,7 @@ class SsaBuilder
 
         // Handle catch variables
         if ($stmt instanceof Stmt\Catch_) {
-            if (is_string($stmt->var->name)) {
+            if ($stmt->var && is_string($stmt->var->name)) {
                 $varName = $stmt->var->name;
                 $ssaId = $this->allocateSsaId();
                 $ssaVar = new SsaVar($ssaId, $varName, 0);

src/gen_stub.php

@@ -3258,7 +3258,11 @@ class PropertyInfo extends VariableLike
         $zvalName = "property_{$propertyName}_default_value";
         if ($this->defaultValue === null && $this->type !== null) {
             $code .= "\tzval $zvalName;\n";
+            if ($this->flags & Modifiers::READONLY || $this->classFlags & Modifiers::READONLY) {
+                $code .= "\tZVAL_UNDEF(&$zvalName);\n";
+            } else {
                 $code .= $this->getTypeDefaultValueCode($zvalName);
+            }
         } else {
             $code .= $defaultValue->initializeZval($zvalName, varName: $this->name->__toString());
         }