Handling of variant tax categories when deleted

[dacook]

Description

OFN CA have raised an issue where the new bulk edit products screen crashes on products that have a deleted tax category.

Discussed further in #product-circle

Currently, you can (soft-)delete a tax category while it is still attached to variants, which might not be a good idea.

Expected Behavior

TBC, see possible fixes below.

Actual Behaviour

The new bulk edit products screen, and some old reports can't handle it and raise an error.

I guess all other product screens handle this gracefully.

Steps to Reproduce

Delete a tax category that has a variant attached to it.

Animated Gif/Screenshot

See Slack thread above

Workaround

Severity

Possible Fix

1. Perhaps we want to nullify and simply show as "None" there (discussed in this PR). But the deletor won't know which variants they have affected.
2. Or I think better, we Prevent delete of tax category if variants are still using it. This might require that the affected variants are shown to the deletor so they can decide what to do.

[dacook]

The problem with the BUU products screen crashing may be solved by #12333