Feedback Better null check for AssetLabelReference

Currently, if you serialize AssetLabelReference as field inspector, and forgot to set it to an actual label, the only valid way to check null appears to be

label.labelString == ""

, using

label.labelString == null

or

label == null

doesn't stop empty label field.

 

two notes:
1. in the upcoming release, we do add a `RuntimeKeyIsValid` method to both `AssetLabelReference` and `AssetReference`, which should solve your problem.
2. in general, if working with strings, I'd always recommend using `string.IsNullOrEmpty` for validation. Won't need that here, just a general note.