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

• Clarify FEEL null handling in FilterExpression dmn-tck/tck#536 ➡️ Clarify FEEL null handling in FilterExpression dmn-tck/tck#536 (comment), test(feel): verify non-member filter expression handling dmn-tck/tck#540
• Clarify FEEL null handling in PathExpression dmn-tck/tck#537 ➡️ Clarify FEEL null handling in PathExpression dmn-tck/tck#537 (comment), test(feel): verify non-existing member access dmn-tck/tck#541
• Clarify mixed type workings of + operator dmn-tck/tck#538 => Works as specified. String cohersion currently not covered by the spec. Created a JIRA issue with the RFP to drive this forward
• Clarify null handling in FunctionInvocation dmn-tck/tck#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): dmn-tck#536 (comment).

[nikku]

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

[nikku]

With dmn-tck#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.