Null geometry clarification

[cholmes]

Related Issue(s): #843

Proposed Changes:

1. Tweaked item spec to not require bbox if geometry is null
2. Updated best practices to explain cases when this should be used.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.

[cholmes]

@m-mohr - could you update the schema to make the bbox not required if geometry is null? And a review would be great.

[m-mohr]

Should we also add an example? And may it only be for checking that the JSON Schema works...

[cholmes]

As for an example, I'm of two minds. 1) be able to make sure the schema works is definitely a good thing. 2) don't really want to highlight this so much. And the plan is to slim down our examples more, so if one of just a handful of examples has a null then it would almost draw attention to it.

If you want to I'd say go for it, as I'm conflicted on it.

[m-mohr]

Let's see whether someone provides a good example: #843 (comment)

[m-mohr]

I added the JSON Schema changes.