Enhance payment form submission by disabling empty fields

[AW-95]

FIX|Fix submission of empty payment amounts in payment form

When submitting the payment form, all input fields (including empty invoice amounts) are sent to the server.
This can lead to unnecessary or incorrect processing of empty values, especially in cases with multiple invoices or multicurrency fields.

This patch ensures that:

• "remain" and "multicurrency_remain" fields are always disabled before submission
• "amount" and "multicurrency_amount" fields are disabled if they are empty

As a result, only relevant (non-empty) invoice payment amounts are submitted.

Technical details

A jQuery handler is added on form submission to disable unnecessary inputs:

• Prevents empty values from being posted
• Reduces payload size
• Avoids potential backend inconsistencies

Before

All fields were submitted, including empty ones.

After

Only non-empty amount fields are submitted.

[ALIT95]

@eldy done please check