Investigate and accommodate the possibility of a missing URI

[bskinn]

Change is here: sphinx-doc/sphinx@13ca232

Issue prompting the change is here: sphinx-doc/sphinx#7095

Will need to add an inventory to tests/resource with missing uri, at minimum.

• Code
• Tests
  • The objects_jsonschema.inv has an index object that has no uri
• Docs
  • Handled a/o 6632dd4
• CHANGELOG