0.00 tax added for financial types with no tax account

Created by: agileware-fj

Steps to reproduce:

1. Turn on Tax & Invoicing then set up a financial type with no tax account - call this e.g. "No Tax calculation"
2. Create a contribution with a single line item using "No Tax calculation" as the financial type.
3. Check the line items using SQL - tax_amount is NULL - correct
4. Edit the Contribution and change a line item
   • Note that tax_amount is 0.00 on this form.
5. Check the line items using SQL - tax_amount is 0.00 - this causes problems with the invoice template ("% GST" with no number) and also account syncing with Xero, which will receive the full amount as tax exclusive (due to 0.00 as a string not being "empty") and add tax on top for accounts tax instead of calculating the percentage of the given amount which is already tax.

Ideal solution – When there is no tax account for the selected financial type,

1. Tax amount field is should be hidden on the line item editor, and
2. Saving the line item should result in the tax_amount column containing NULL

We can work on a solution for this if needed.