Improve handling of rdf:nil when using not() WOQL construct with sys:subdocument

[hoijnet]

Describe the bug
There is a bug where operations with the not() operator does not behave according to expectation.

When querying for document, I often want to select only documents, not subdocuments. I often forget that rdf:nil is not treated the same way when using the not() construct.

Here are the four cases, of which I think the last one misbehaves, and should work the same as number 3. My reasoning is that the handling in example 2 and example 4 should work the same way, just in the inverse. The reason I think that this is a bug, at least it goes against expectations, when running example 1, the v:unbound variable is set to rdf:nil.

1. Find all subdocuments, returns expected documents (with unbound variable)

and(
  triple("v:doc","rdf:type","v:type"),
  quad("v:type","sys:subdocument","v:unbound","schema")
)

2. Find all subdocuments, returns expected documents (with rdf:nil)

and(
  triple("v:doc","rdf:type","v:type"),
  quad("v:type","sys:subdocument","rdf:nil","schema")
)

3. Find all documents (i.e. not subdocs), returns expected documents (with unbound variable)

and(
  triple("v:doc","rdf:type","v:type"),
  not().quad("v:type","sys:subdocument","v:unbound","schema")
)

4. Find all documents (i.e. not subdocs), does not return expected documents (with rdf:nil)

and(
  triple("v:doc","rdf:type","v:type"),
  not().quad("v:type","sys:subdocument","rdf:nil","schema")
)

To Reproduce
Steps to reproduce the behavior:

1. Create a subdocument type
2. Create a data product with a type that inherits from another type, and put the subtype as a mandatory property.
3. Create documents to test with
4. Try examples 1, 2, 3, 4
5. Notice how example 4 does not return document references

Expected behavior
The expectation is that the result of example 3 and example 4 should be the same.

Screenshots
N/A

Info (please complete the following information):

• TerminusDB version: 11.1.9 running through https://DFRNT.com

Additional context
N/A