[Property Editor] Make it clear if a value is expression, or if it's null because it's optional

Currently, we don't display expressions differently than literal values. So a user might see a string input with `title` and not know if `title` is the variable name or the literal string.

Similarly, an input might say `null`, but that could mean the default value is `null` or that its optional and there is no value.

It would be nice to figure out how to distinguish between these cases in the UI.

首先在 DevTools 仓库中找到 Property Editor 的实现，并检查当前如何表示表达式、字面量值和可选的 null 值。在更改 UI 之前，明确预期的视觉区分和验收情况；完成的标准是用户能够区分这些情况。

由索引模型根据 Issue 内容生成。