[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.

Start by locating the Property Editor implementation in the DevTools repository and reviewing how expressions, literal values, and optional null values are currently represented. Clarify the intended visual distinction and acceptance cases before changing the UI; done means users can tell these cases apart.

Written by the indexing model from the issue text.