client: imports: avoid error when cron string cannot be parsed when trying to edit imports To prefill the form fields that generate the cron string, the currently stored cron string needs to be parsed back. This parsing only handles numerical values for each of the six cron fields. In case of something like */15 the regular expression does not match, leading to a null value. The cronMode select now has an empty option for cases where the cronString does not match any known pattern.