Encoding of non-existing info in XML

[Camille Laibe]

I think when data is not available, we should not add it in the XML. For example in http://wwwdev.ebi.ac.uk/biomodels/model-repository/model/50?format=xml the publication is recorded as: <publication>Not Provided</publication> To make processing of the XML more automatic, it maybe advisable to just not add this info in this case.