TraceQL: make nil type symmetric

[electron0zero]

What this PR does:
makes the nil symmetric.

Which issue(s) this PR fixes:
Fixes #3989

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[electron0zero]

After some digging into the failing test, I have arrived at a question that boils down to, “should a query like nil != 0 return data or not?”

I’m unclear about the expected behavior for queries nil != 0, or nil != 5, should they evaluate to true or false?. This behavior isn’t currently defined, and we need to establish a clear definition.

Even if we prevent queries TypeNil != TypeInt or TypeNil != TypeFloatat the parse layers, users can write queries that end up evaluating uptoTypeNil != TypeInt`.

For instance, avg(.foo) (foo doesn't exist) is not evaluated to nil. so a query like { } | avg(.dne) != 0 will finally be evaluated to nil != 0

We introduced the nil type to allow easy checks for non-existent attributes using queries like { .foo != nil }. If we want to maintain symmetry and allow queries like { nil != .foo }, we need to clearly define the behavior of nil.

Also a follow up question: should functions be evaluating to nil, should we be evaluating avg(.foo) to nil, if foo doesn't exist?