[15.0][FIX] Clear price_tax when the Taxes field is empty

[newtratip]

I found an error when creating a journal entry for an expense without taxes while the price_tax field still has a value. On the UI, the price_tax field is hidden, so the issue is not visible to the user until the journal entry is created

This PR resets price_tax to 0 when no taxes are set.

Steps to reproduce

1. Create an Expense Report and add an expense line with taxes (Example: amount = 100, taxes = 7%, price_tax = 7)
2. Remove the 7% tax and save the expense sheet. The price_tax value remains 7 (it is not cleared to zero)
3. Submit the expense sheet to create the journal entry. The system throws an error due to a tax difference, as shown in the screenshot below.

@Saran440, Please consider reviewing and approving this PR.

[OCA-git-bot]

Hi @ps-tubtim,
some modules you are maintaining are being modified, check this out!

[Saran440]

LGTM 👍