Checkmk Werk 12619: Fix error on editing downtimes without specifying a new downtime start

ID: 12619 Title: Fix error on editing downtimes without specifying a new downtime start Component: Multisite Level: 1 Class: Bug fix Version: 2.1.0i1 If you edited a downtime and used the option "Change downtime end" without also specifying "Change downtime start", an internal error "'<' not supported between instances of 'NoneType' and 'int'" occurred.