Prevent Collection from adding 'rdf:nil rdf:rest rdf:nil.' triples

[progval]

Summary of changes

Creating 'Collection(graph, RDF.nil)' used to add a 'rdf:nil rdf:rest rdf:nil.' triple to the graph, which turned the empty list into an infinite list.

This was a side-effect of unconditionally appending 'seq' to the collection, which is not possible when the collection is 'nil' without ill side-effects.

This commit makes the append conditional; and also checks the collection is non-empty in case the user explicitly tries to add item to the empty list

Checklist

• [x] Checked that there aren't other open pull requests for the same change.
• [ ] Checked that all tests and type checking passes.
• If the change has a potential impact on users of this project:
  • [x] Added or updated tests that fail without the change.
  • [ ] Updated relevant documentation to avoid inaccuracies.
  • [x] Considered adding additional documentation.
• [x] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.635%. first build when pulling 6e65136305158bb845887e03561ac905166b4d23 on progval:collection-empty-list into 46695eb291adeba5e112f92e0bcd9d8ed1c35243 on RDFLib:main.

[ashleysommer]

There's an odd failing test now, weirdly its the same SPARQL test that is failing in #2796 that is completely unrelated to this PR.

[ashleysommer]

That sporadic test is no longer failing, this can merge.