Preprocessor to fix `constant is null` and `null is null`

[baseTwo]

In the ExpressionToCSharpConverter there is the code to fix constant is null and null is null, however this needs to be moved to a pre-processor.

          Create an issue to move this to a  pre-processor traverser

Originally posted by @baseTwo in https://github.com/FirelyTeam/firely-cql-sdk/pull/616#discussion_r1810532200