From my various tests 
: the tags key in file3 :
---
tags:
---
… corresponds to null.
While the tags key in file4:
---
tags: []
---
… corresponds to blank.
So while they both appear as Empty in Properties in Live Preview, they’re actually not the same underneath.
The difference can be seen when you search for the tags property 
You could try to play around with an is empty condition (which I think handles both null and blank values now … but I think it also returns files where the key doesn’t exist so you might need to filter out the unwanted files using something like: note.keys().contains("tags"))
This could also be useful: Bases Migration/Quick Start Guide