label:assets aren't required in links #183
Description
Describe the bug
The label extension spec says:
applies to situations where the labels may apply to certain assets inside the source imagery Item, but not others (e.g. if the labels were traced on top of RGB imagery, but the source item also contains assets for a Digital Elevation Model).
This is not going to apply to every label item, but the field is required on the LabelLinkExtension.
Expected behavior
stac4s doesn't think source links on label items without a label:assets property are invalid