[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.