Clarify FEEL null handling in PathExpression

[nikku]

Describe the context

The DMN 1.4 specification gives users some pointers on how FEEL should handle invalid inputs and data formats (cf. additional context). From what it reads to me the way is null cohersion. Invalid (incompatible operations, access, or invocations) yield null. That intention is not explicitly expressed though (you need to read between the lines, interpret what is actually specified.

null cohersion around PathExpression is not clearly specified and may lead to engines behaving slightly different:

// given
context = {
  a: {
    b: 3
  }
}

// when
c = context.a.c

Expected Behavior

// then (?)
c = null

// but also transitive (?)
c.foo = null

// also works with `get value` (?)
get value(a, "c") = null

Alternative Interpretations

To be clear there exists a number of different possible interpretations:

strict null handling, blowing up

c is not present on context `{ b }`

Additional Context

I'm aware of the following cases of null cohersion, explicitly defined by the spec:

Access function with wrong arguments

date(1, 2, 3, 4, 5, 6) = null

Unboxing of single value lists

["a"] + "a" = "aa"

---

Related to #536

[nikku]

This seems to be clarified by @barmac in #536 (comment):

{ b: 1 }.a < 3

// which is equal to
null < 3

What is missing is TCK test cases to clearly document the behavior.