fix: Add shipping charges to taxes only if applicable

[deepeshgarg007]

Recently server-side implementation for shipping rule was added. For old invoices generated before this change following error was seen when making a subsequent document (Sales Invoice, Delivery Note, etc) from it.

 File "apps/erpnext/erpnext/controllers/taxes_and_totals.py", line 368, in get_current_tax_amount
  self.set_item_wise_tax(item, tax, tax_rate, current_tax_amount)
 File "apps/erpnext/erpnext/controllers/taxes_and_totals.py", line 376, in set_item_wise_tax
  if tax.item_wise_tax_detail.get(key):
AttributeError: 'NoneType' object has no attribute 'get'

[deepeshgarg007]

@Mergifyio backport develop

[mergify]

backport develop

✅ Backports have been created

• #29501 fix: Add shipping charges to taxes only if applicable (backport #29428) has been created for branch develop