Adds a default return value to the generate_license_data_by_id function which returns data for a CC0 licence if none is found, or the found license object has null values (for old hepdata entries). Also adds testing for this. closes #792
coverage: 83.309% (-0.1%) from 83.41%
when pulling eafb5da0358962e14e709dfaf0f19fe0cf2a1f26 on display_default_license
into 6702cefbdcf1012fce47477eab94881f26c0790e on main.
Adds a default return value to the generate_license_data_by_id function which returns data for a CC0 licence if none is found, or the found license object has null values (for old hepdata entries). Also adds testing for this. closes #792