Correctly transform exists to is not null check

Reviewers: buda, mislav.bradac

Reviewed By: mislav.bradac

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D731

tests/public_benchmark/ldbc/test_cases/queries/short_query_2.oc

@@ -3,7 +3,7 @@ MATCH (:Person {id:"933"})<-[:HAS_CREATOR]-(m)-[:REPLY_OF*0..]->(p:Post)
 MATCH (p)-[:HAS_CREATOR]->(c)
 RETURN
   m.id as messageId,
-  CASE m.content IS null
+  CASE m.content IS NOT null
     WHEN true THEN m.content
     ELSE m.imageFile
   END AS messageContent,

tests/public_benchmark/ldbc/test_cases/queries/short_query_4.oc

@@ -1,7 +1,7 @@
 // Message Content
 MATCH (m:Message {id:"1236950581249"})
 RETURN
-  CASE m.content IS null
+  CASE m.content IS NOT null
     WHEN true THEN m.content
     ELSE m.imageFile
   END AS messageContent,