No tax should apply Tax basis = Store & store zone has no tax set

[piloujp]

Fixes #6384

[drbyte]

I don't think this fixes the root problem.

This PR will simply apply no tax, instead of reverting to the store's tax zone.

[piloujp]

ZC is actually programmed to apply store tax (When tax basis is 'Store') only if client is in same zone. Then if client is in another zone, it applies client's zone tax (actually with error in calculation). If it is meant to be like this, then setting tax basis to store is exactly the same as setting it to shipping.
I did this PR so it applies no tax when client zone is different than store one. Another possibility would be to apply store tax whatever client zone is.
When store has no tax zone, no tax is applied either.

Sorry, but I don't understand when you write instead of reverting to the store's tax zone., reverting from which situation are you referring to?