Undefined index error when using exceptions that do not have settings

Problem/Motivation

If you have exceptions from previous versions and update to the latest, there is an undefined index error.

Steps to reproduce

Have exceptions days from a previous version and update to the latest, then try and view a page that has office hours, the following error is thrown:
Error: Unsupported operand types in Drupal\office_hours\Plugin\Field\FieldFormatter\OfficeHoursFormatterBase->mergeDefaults() (line 105 of office_hours/src/Plugin/Field/FieldFormatter/OfficeHoursFormatterBase.php).

Proposed resolution

Will provide patch to fix.