Clarify `null` handling in different cases

[nikku]

What shall we do

Related to this conversation we should clarify the handling of null in DMN FEEL.

Open questions

• [x] https://github.com/dmn-tck/tck/issues/536 :arrow_right: https://github.com/dmn-tck/tck/issues/536#issuecomment-1385863569, https://github.com/dmn-tck/tck/pull/540
• [x] https://github.com/dmn-tck/tck/issues/537 :arrow_right: https://github.com/dmn-tck/tck/issues/537#issuecomment-1386634436, https://github.com/dmn-tck/tck/pull/541
• [x] https://github.com/dmn-tck/tck/issues/538 => Works as specified. String cohersion currently not covered by the spec. Created a JIRA issue with the RFP to drive this forward
• [x] https://github.com/dmn-tck/tck/issues/539

[nikku]

As discussed with @barmac he plans to bring attention to these issues in the DMN 1.5 RTF call later today.

[nikku]

I've added TCK test cases for the most important cases (filter and path expression null handling): https://github.com/dmn-tck/tck/issues/536#issuecomment-1396591619.

[nikku]

Will follow up with function invocation as I find the time. Let's get all aspects of null handling verified by the TCK :rocket:

[nikku]

With https://github.com/dmn-tck/tck/issues/539 clarified + additional tests merged this issue is closed.

[nikku]

TLDR: DMN FEEL works as I assumed, with invalid conversions / operations resulting in null and an errorResult indicating that there was an undesired conversion.