Fix New Account Wizard throws exception on empty payment method selected

Summary:
"Invalid payment type for schedule" was thrown when the user proceeded
through Credit Card Schedule Page leaving empty payment method
(default).

This commit:
* sets default credit card payment method to "Direct debit",
* fixes incorrect Next button tooltip messages on the Schedule page.

BUG: 421569
FIXED-IN: 5.0.9

Reviewers: tbaumgart

Reviewed By: tbaumgart

Subscribers: ostroffjh

Tags: #kmymoney

Differential Revision: https://phabricator.kde.org/D29776