Fix PHP crash when copying course with OnlyOffice objects

[jheim2]

Set $title to an empty string if the "upload_files" key is missing from the $_POST array, preventing the PHP script from crashing when copying a course with OnlyOffice objects.

[rfalkenstein]

Some more context for reproduction:
The error report we receive is the following: "Whoops\Exception\ErrorException thrown with message "Trying to access array offset on value of type null".

#22 Whoops\Exception\ErrorException in /path/to/webroot/Customizing/global/plugins/Services/Repository/RepositoryObject/OnlyOffice/classes/class.ilObjOnlyOffice.php:79
#21 ilErrorHandling:handlePreWhoops in /path/to/webroot/Customizing/global/plugins/Services/Repository/RepositoryObject/OnlyOffice/classes/class.ilObjOnlyOffice.php:79
#20 ilObjOnlyOffice:doCreate in /path/to/webroot/Services/Object/classes/class.ilObject2.php:55
#19 ilObject2:create in /path/to/webroot/Services/Object/classes/class.ilObject.php:1600
#18 ilObject:cloneObject in /path/to/webroot/Services/Object/classes/class.ilObject2.php:129
#17 ilObject2:cloneObject in /path/to/webroot/webservice/soap/classes/class.ilSoapUtils.php:409
#16 ilSoapUtils:cloneNode in /path/to/webroot/webservice/soap/classes/class.ilSoapUtils.php:264
#15 ilSoapUtils:ilClone in /path/to/webroot/webservice/soap/include/inc.soap_functions.php:464
#14 ilSoapFunctions:ilClone in /path/to/webroot/webservice/soap/classes/class.ilSoapUtils.php:356
#13 ilSoapUtils:callNextNode in /path/to/webroot/webservice/soap/classes/class.ilSoapUtils.php:265
#12 ilSoapUtils:ilClone in /path/to/webroot/webservice/soap/include/inc.soap_functions.php:464
#11 ilSoapFunctions:ilClone in /path/to/webroot/Services/Container/classes/class.ilContainer.php:555
#10 ilContainer:cloneAllObject in /path/to/webroot/Services/Object/classes/class.ilObjectCopyGUI.php:1071
#9 ilObjectCopyGUI:copyContainer in /path/to/webroot/Services/Object/classes/class.ilObjectCopyGUI.php:920
#8 ilObjectCopyGUI:copyContainerToTargets in /path/to/webroot/Services/Object/classes/class.ilObjectCopyGUI.php:138
#7 ilObjectCopyGUI:executeCommand in /path/to/webroot/Services/UICore/classes/class.ilCtrl.php:203
#6 ilCtrl:forwardCommand in /path/to/webroot/Modules/Category/classes/class.ilObjCategoryGUI.php:196
#5 ilObjCategoryGUI:executeCommand in /path/to/webroot/Services/UICore/classes/class.ilCtrl.php:203
#4 ilCtrl:forwardCommand in /path/to/webroot/Services/Repository/classes/class.ilRepositoryGUI.php:243
#3 ilRepositoryGUI:show in /path/to/webroot/Services/Repository/classes/class.ilRepositoryGUI.php:223
#2 ilRepositoryGUI:executeCommand in /path/to/webroot/Services/UICore/classes/class.ilCtrl.php:203
#1 ilCtrl:forwardCommand in /path/to/webroot/Services/UICore/classes/class.ilCtrl.php:178
#0 ilCtrl:callBaseClass in /path/to/webroot/ilias.php:24

We can reproduce this error not only when copying a course with an OO object but also when copying an OO-object.