l10n_it_withholding_tax: avoid print net pay residual if invoice or credit note hasn't withholding tax

[patrickt-oforce]

Module

l10n_it_withholding_tax

Describe the bug

Actually, when an invoice or credit note is printed and has payment, the amount of net pay residual is printed even f no withholding tax is present in document

To Reproduce

Affected versions:

• 16.0 - [16.0] [IMP] l10n_it_withholding_tax: avoid to print net pay residual if invoice or credit note hasn't withholding tax #5279
• 18.0 - ?

Steps to reproduce the behavior:

1. Create an invoice add a line, without withholding tax
2. Confirm it
3. Print, you should view the net pay residual amount in PDF

Expected behavior
If not withholding tax, avoid print amount of net pay residual