Make chromosomes optional

[egonw]

Fixes #2796

Description

The previous query requires genes to be always linked to chromosomes, but SARS-CoV-2 does not have a chromosome.

Caveats

Check any of the following which apply:

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
  • I have made corresponding changes to the documentation
• This change requires new dependencies (please list)

Testing

Provide instructions, so we can reproduce. Please also list any relevant details for your test configuration.

• Test if genes show up in the Genome panel at https://qlever.scholia.wiki/taxon/Q82069695#genome

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have not used code from external sources without attribution
• I have considered accessibility in my implementation
• There are no remaining debug statements (print, console.log, ...)

[egonw]

Fail with Python 3.10 probably related to #2791