[v3.4] Fix Taxon taxonomy id validation regression

[RyanofWoods]

Summary

Fixes #5187.

In v3.4 I added optional Taxon validations which become mandatory in v4.0.
This caused a regression which made all root taxons with children always invalid.

Fix for v4:
#5189

This optional validation for root taxons (parent_id is nil), was
checking for uniqueness of taxonomy_id among all Taxons. This is wrong
however, as the children Taxons of this root Taxon will point to the
same Taxonomy. Instead, we should only look among other root Taxons.

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[elia]

@RyanofWoods so if I understand correctly the if check deceived all of us because it looked like it was applied to the query, while it wasn't, right?

[RyanofWoods]

Correct @elia, the if check may have caused some to think that only root Taxons are looked at for what taxonomy_ids are used. However, instead, it's only used to determine if this validation should be triggered or not. It doesn't affect the scope/query, and the uniqueness scope is needed for this.

I hope my explanations have been clear.