change transactions clients method signatures to match use, allow result of None to return 204

[philvarner]

Related Issue(s):

• TransactionsClient should provide option for not returning an Item from endpoints #376

Description:

• for all Txn clients, update the method return types to match what can actually be returned. This is now None, a STAC object type, or a starlette Response.
• In addition to the existing behavior of returning a 200 + STAC object type or Response, also return a 204 No Content if the result value is None.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[moradology]

Discussed this with @geospatial-jeff. On the one hand, it is generally the case that exceptions are used to inject non-200 responses. At the same time, we agreed it felt bizarre to force exceptions in non-exceptional cases. +1