Allow G123 to run without phasing analysis when no default phasing exists

[SKM2227229725]

Fixes #795

Summary

Previously, g123_gwss() required a default phasing analysis ID,
which caused an assertion error for datasets such as amin1 and adir1
where no phasing analysis is available.

What this PR changes

• Allows sites=None when no default phasing analysis exists
• Adds None to valid site options
• Maintains backward compatibility for datasets that do use phasing
• Avoids the need for placeholder strings like "noneyet"

Rationale

G123 uses unphased genotype data for the main calculation.
Requiring a phasing analysis ID should not be mandatory for
datasets that do not provide phasing data.

This change enables G123 to run cleanly on such datasets
without breaking existing behaviour.

[jonbrenas]

I don't think it does what you say. It still uses the _default_phasing_analysis, which is a phasing analysis and thus excludes unphased data.

[SKM2227229725]

@jonbrenas Thanks for the review and for pointing this out. I will re-check the implementation to ensure that G123 can run without relying on _default_phasing_analysis when no default phasing analysis exists. I will update the PR accordingly.

[SKM2227229725]

Thanks for the review. I will work on the requested changes. Please assign this to me.