Update Providers_model.php

[tm8544]

To handle deprecations on lines 527 and 529.
strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated
When there is a full-day exception, $working_plan_exception['start'] and $working_plan_exception['end'] are both null

This is an issue that came up on #1497 and at some point it seemed to be cleared, so I closed #1497.
Now it is back, and now I can explain more precisely the conditions it occurs.

First of all it is not an exception but a deprecation, but if handled now there would be no exceptions later on with future php versions.

This appears under the following conditions:

1. in config.php there is const DEBUG_MODE = true; and
2. error_reporting in php.ini has been set to report deprecations.

Maybe this is the reason you did not see this. In my case both conditions were met.

[alextselegidis]

Hello!

Thanks for this PR.

I see the problem but it looks like this one requires some additional handling.

I will take a closer look.

Alex Tselegidis, Easy!Appointments Creator Need a customization? Get a free quote!

[alextselegidis]

Added check for this case

Alex Tselegidis, Easy!Appointments Creator Need a customization? Get a free quote!