"Created at" responses should not have a response body

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

Currently the "Created at" responses from the Management API contain a response body with "null". Turns out the built-in CreatedAtActionResult is built upon ObjectResult, which outputs null values in the response body.

The "null" response is messing with the new back-office client.

To work around this, a new EmptyCreatedAtActionResult has been added to the Api.Common, and this is then used for all "Created at" responses created by Management API base controller.

The EmptyCreatedAtActionResult is purposely placed in Api.Common and made public (sealed), so it can be reused in other APIs.

Testing this PR

"Created at" responses are built in different ways depending on the action performed. To test this PR, verify that the following API operations return a HTTP201 status code, and that the response a correct location header:

• Creating a new script.
• Creating a new data type folder.
• Creating a new saved search.

[nikolajlauridsen]

Looks good 😄 👍