Server should log a WARN in case of missing export metadata instead of throwing an exception

[qnoid]

The BuildMessageBodyReader class that is responsible for reading the export metadata in the 3rd stage of uploading an export is currently throwing a java.lang.ClassCastException: javax.json.JsonValueImpl cannot be cast to javax.json.JsonString when a metadata field is not present.

This leads to a 500 server error. Instead it should default to an empty string, log the error as WARN (so that an alert is created and notified) and complete the upload as normal.