`blank` and `empty` special keywords are not properly parsed in Liquid 5

Hey friends,

I was just working on upgrading Jekyll from Liquid v4 to v5. I have everything working except our where filter. We have our own where filter for arcane historical reasons. It works flawlessly in Liquid 5 except in two cases: using empty or blank as the value comparator:

{{ hash | where: "property", empty }}
{{ hash | where: "property", blank }}

We expect these to somewhat behave like Ruby's empty? and Rails's blank? methods, since previously Liquid would pass us the Liquid::Expression::MethodLiteral corresponding to these special keywords. We could then process them specially. Unfortunately, we're now just getting the empty string ("") when we use blank or empty. (https://github.com/jekyll/jekyll/pull/9030#issuecomment-1094510678)

I see that @dylanahsmith (👋) wrote #1300 which changed Liquid::Expression::LITERALS to set empty and blank as "" which could be related to why our filter is getting target = "" instead of target = Liquid::Expression::MethodLiteral<name=:empty? to_s="">.

Is there any way we, as the filter authors, could be notified that the target is the special keyword empty or blank?

Thanks for your hard work on Liquid!

Start by reading the behavior described around Liquid::Expression::LITERALS and the change in #1300, then trace how the parser passes empty and blank to filters. Compare that with the prior MethodLiteral behavior referenced in the issue and the custom where filter example. Done means filter authors can distinguish these special keywords from an ordinary empty-string comparator, with coverage for both cases.