Prevent NULL values from being passed to string functions

Problem/Motivation

There are several places in the module where possibly NULL values are passed to string functions, which will throw an error.

Steps to reproduce

This affects developers building functionality on top of the module that creates events programmatically, rather than events created through the UI.

For example, if an event series is created programmatically with daily recurrence and the end_time value is left NULL because the event is using duration instead—which would seem to be a legal thing to do, since end_time is not a required field—then passing this event to DailyRecurringDate::convertEntityConfigToArray will raise an error when it tries to call $config['end_time'] = strtoupper($event->getDailyEndTime());

Proposed resolution

Protect possibly NULL values with the null-coalescing operator when passing to string functions. For example, strtoupper($event->getDailyEndTime() ?? '');