datacite: write license information in XML metadata

[GraemeWatt]

The license information is currently written for data tables:

hepdata/hepdata/modules/records/templates/hepdata_records/formats/datacite/datacite_data_record.xml

Lines 47 to 51 in 50a2498

	{% if license %}
	<rightsList>
	<rights rightsURI="{{ license.url }}">{{ license.name }}</rights>
	</rightsList>
	{% endif %}

and resource files:

hepdata/hepdata/modules/records/templates/hepdata_records/formats/datacite/datacite_resource.xml

Lines 42 to 46 in 50a2498

	{% if license %}
	<rightsList>
	<rights rightsURI="{{ license.url }}">{{ license.name }}</rights>
	</rightsList>
	{% endif %}

only if an explicit license is given (if license). The XML metadata should be modified to write the default CC0 license otherwise, since this is now rendered on the HTML pages by default.