Mark arena as nullable

`CustomListValue::CustomListValue()` set's it into nullptr and it triggers
`-fsanitize=nullability`.

There are not deference of this pointer, it used only in comparison. Marking it
`nonnull` does not help to protect dereferences.

Also mark `arena` as const, to make sure it's not used for allocation, those
methods are non-const.

PiperOrigin-RevId: 801935442

Author: CEL Dev Team

common/values/custom_list_value.h

@@ -241,7 +241,7 @@ class CustomListValueInterface {
 
   struct Content {
     const CustomListValueInterface* absl_nonnull interface;
-    google::protobuf::Arena* absl_nonnull arena;
+    const google::protobuf::Arena* absl_nullable arena;
   };
 };