Fix Missing startTime Metadata Field Exception

[KeksBombe]

Fixed a critical bug where the OpenCast plugin would throw an xoctException with the message "could not find metadata field with id startTime" when uploading content with preview images enabled but timestamp disabled.

Problem
When creating a regular upload event (not scheduled), the startDate and startTime metadata fields are not automatically added to the metadata catalogue. However, the MetadataToXML transformer assumed these fields would always be present, causing the plugin to crash during the upload process.

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Fixed a critical bug where the OpenCast plugin would throw an Exception when trying to delete a publication.

Problem
When trying to delete something the HeaderText was not Set.

Stack trace location:

Solution
Modified the MetadataToXML::getXML() method to handle missing startDate and startTime fields gracefully:

Added try-catch blocks around metadata field access
Provides sensible fallback values:
startDate: Current date if field is missing
startTime: 00:00:00 if field is missing
Maintains backward compatibility with existing scheduled events that have these fields
Changes Made
File: src/Util/Transformator/MetadataToXML.php
Added exception handling for missing startDate field (fallback to current date)
Added exception handling for missing startTime field (fallback to 00:00:00)
Added import for xoctException class