Incorrect isEmpty() check for some empty Exception days

Problem/Motivation

The isEmpty method (\Drupal\office_hours\Element\OfficeHoursDatetime::isEmpty) fails on a -1 value. It checks for a string '-1' but sometimes it gets the -1 integer and doesn't mark that as empty.

Steps to reproduce

(using the dev release) I added a field to a node, with the exceptions. After adding an item and exception, I re-edited the node and removed the exception. Upon saving, there are a lot of form-errors: This value should be of the correct primitive type.

Proposed resolution

Change the check to if ((int) $element === -1) { ... }

Remaining tasks

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User interface changes

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API changes

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Data model changes

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