improve LicenseUrl build in json creation

[dthiex]

Currently we create a LicenseUrl link object no matter weather LicenseUrl is defined in the yaml or not. This leads to an empty LicenseUrl as can be seen here: https://collections.eurodatacube.com/stac/tsmn_water_quality_saturday.json

We should only include this in the json if LicenseUrl is defined. Main difficulty here is to format the json correctly so that the comma before the LicenseUrl link object is placed if it's present and not if it's not.

Needs to be done here: https://github.com/eurodatacube/public-collections/blob/main/_build/stac.hbs