Fix for caosdb-server#154

Summary

Fix for caosdb-server#154 (closed)

Focus

Bug was in apply POV: If the operator is '!=' we need to check for "NOT EXISTS", while "EXISTS" corresponds to the "=" operator.

Test Environment

Test are in caosdb-pyinttest!13 (merged)

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CHANGELOG.md

@@ -22,6 +22,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed ###
 
+* Bug in applyPOV which led to caosdb-server#154: Queries with "!=" returned
+  the same results as "=" (for references).
+
 ### Security ###
 
 ## [4.1.0] - 2021-06-11 ##