[Bug] [API] CreateIssue not ignore empty assignees

[6543]

afected version: 1.11-dev
not afected: 1.9.x; 1.10.0

If json include "assignees": [ "" ], responce is 422:

{
  "message": "Assignee does not exist: [name: user does not exist [uid: 0, name: , keyid: 0]]",
  "url": "https://gitea.com/api/swagger"
}

What sould happen: ignore assignees