prevent php 8.1 deprecation notice in strtotime

[peterjaap]

I got this error;

Fix proof:

php > $data['date'] = null;
php > $dateTime = date('d-m-Y H:i', strtotime($data['date']));
PHP Deprecated:  strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated in php shell code on line 1
PHP Stack trace:
PHP   1. {main}() php shell code:0
PHP   2. strtotime($datetime = NULL) php shell code:1

Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated in php shell code on line 1

Call Stack:
   32.4666     603888   1. {main}() php shell code:0
   32.4666     603888   2. strtotime($datetime = NULL) php shell code:1

php > $dateTime = date('d-m-Y H:i', strtotime($data['date'] ?? ''));
php > var_dump($dateTime);
php shell code:1:
string(16) "01-01-1970 00:00"

Resolves #739