[(get-else $ ?e ?attr ?non-nil-sentinel-value) ?captured-value]

or (or-join [?e ?v] [?e ?a ?v] (and (not [?e ?a]) [(ground "non-nil-sentiel") ?v)) for more complex cases

or use rules and make sure one implementation always matches and one never does

(for any given thing you are checking)

note unfortunately you can’t safely have nil values in your intermediate result sets so you usually have to replace it with some non-nil sentinel value

namespaced-keywords work well IME

and then postprocess the query result back to nil if you need it

thx