Don't fail on item.set_self_href(None) for items with relative asset hrefs

[lossyrob]

Description:
Currently if you call set_self_href on an Item with None as the parameter, if that item has assets with relative hrefs, the logic that attempts to ensure relative asset hrefs remain valid when switching the item href ends up throwing an error. This change makes it so that, if you are setting the item self href to None, any relative asset hrefs are left as is.

PR Checklist:

• Code is formatted (run scripts/format)
• Tests pass (run scripts/test)
• This PR maintains or improves overall codebase code coverage.
• Changes are added to the CHANGELOG. See the docs for information about adding to the changelog.