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?

[piloujp]

I thought this PR was an acceptable solution, the only argument against it was cleared in issue #6384 or was it?
To be clear here is what it does depending on zone settings, it is as stated on explanation text for this setting (apply store tax only if client is in same zone):
store zone ---- client zone ----- applied tax
not set -------- not set ---------- none
set ------------- not set ---------- none
not set ---------- set ------------- none
set -------------- set, diff. zone --- none
set -------------- set, same zone -- yes, store tax

Of course it could be modified if requested.
Actually ZC does this;
store zone ---- client zone ----- applied tax
not set -------- not set ----------- none
set ------------ not set ----------- none
not set -------- set --------------- yes, client tax
set ------------ set, diff. zone ----- yes, client tax
set ------------ set, same zone --- yes, store/client tax
Which is exactly identical to 'Basis of Shipping Tax' set to 'Shipping'.