Create clips throws a 503 if a username is sent as broadcaster_id

[Dkamps18]

Brief description
Create clips throws a 503 if a username is sent as broadcaster_id
How to reproduce
Send a username as value of the broadcaster_id parameter rather than a userid
Expected behavior
To throw a 400 rather than a 503.
Screenshots

[jbulava]

Ticketed internally as HLX-1780.

[Dkamps18]

This got fixed to the extend that the endpoint no longer returns a 503 but now returns an empty data array instead of returning a 400

[Xemdo]

Endpoint is now returning HTTP 400 instead of an empty array. Seems to be resolved