Root taxons with at least a child are not valid

[elia]

Solidus Version:
v4

To Reproduce

• Create a taxonomy
• Create a taxon with taxonomy.root as its parent
• Check taxonomy.root.valid?

Current behavior

It returns false

Expected behavior

Should return true

Additional context

Looks like creating products with associated taxons having those conditions might trigger that validation in some cases.

cc @filippoliverani