Return empty body for 204 responses in API

What does this MR do?

Spotted while working on another MR:

Previously we didn't always specify a body for 204 responses, these would just return the .to_s representation of the Grape route block expression, e.g. "<Project:0x0000560fe150ce80>". In some cases we also explicitly return an empty hash {}.

This is now changed to always use the no_content! helper. Since this aborts the execution we also have to tweak the order slightly, especially with the destroy_conditionally! helper.

This shouldn't break any clients since they're not supposed to parse the body from a 204 response, and in most cases this would have already failed since the response wasn't valid JSON.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team