Consider making /api/config/properties return 204 No Content instead of 404 when a setting is missing/empty

[tdonohue]

Describe the bug
This seems to quickly becoming a "frequently asked question" on mailing lists & Slack. Some seem concerned that (when watching UI requests) they sometimes see error responses (404, etc)

For example: /server/api/config/properties/google.analytics.key is requested by the UI to determine if Google Analytics is enabled or not. This returns a 404 if GA is disabled.

To Reproduce
One basic example:

1. Do not set google.analytics.key on backend
2. Visit the UI with your browser's DevTools open. On the Network tab you'll see the 404 above highlighted in red. The browser also lists this 404 response as an "error" in the DevTools Console.

This behavior can be confusing to sites as they may worry something is broken in the UI. But, in fact this is expected behavior when Google Analytics is disabled.

Expected behavior
We should consider whether these 404s should simply become 204. A 204 will not be seen as an error by the user's browser. But, it would also communicate to the DSpace UI that this configuration is not set or has no value on the backend.

This will require discussion, as it would need to be a documented API change.