SEPA NG Payment Processor: Error when price set contains a line with amount = 0.00

(This is a copy of the issue https://github.com/Project60/org.project60.sepa/issues/565 )
We have an event registration with a price set, where one option is free (amount=0.00), whereas another option is payable.

With the NG payment processor, CiviCRM throws an error:

Sorry, due to an error, we are unable to fulfill your request at the moment. You may want to contact your administrator or service provider with more details about what action you were performing when this occurred.
Expected one Contribution but found 25

The "old" payment processor works fine, as well as the PayPal payment processor.

This was reported on CiviCRM: 5.19.4 on Wordpress, CiviSEPA: 1.4
Should be verified with the current version CiviSEPA 1.5 and the (now separate) SEPA PP extension

Start by reproducing the error in org.project60.sepapp with an event registration whose price set includes both a free option and a payable option. Verify the behavior against CiviSEPA 1.5 and the separate SEPA PP extension, comparing the NG processor with the old and PayPal processors. Done means the NG processor handles the zero-amount line without the “Expected one Contribution but found 25” error.