Fix new release creation API to allow empty target (#5870)

* Fix new release creation API to allow empty target

* Add more test cases

* Update swagger

vendor/code.gitea.io/sdk/gitea/gitea.go

@@ -70,6 +70,8 @@ func (c *Client) getResponse(method, path string, header http.Header, body io.Re
 		return nil, errors.New("403 Forbidden")
 	case 404:
 		return nil, errors.New("404 Not Found")
+	case 409:
+		return nil, errors.New("409 Conflict")
 	case 422:
 		return nil, fmt.Errorf("422 Unprocessable Entity: %s", string(data))
 	}
@@ -77,7 +79,9 @@ func (c *Client) getResponse(method, path string, header http.Header, body io.Re
 	if resp.StatusCode/100 != 2 {
 		errMap := make(map[string]interface{})
 		if err = json.Unmarshal(data, &errMap); err != nil {
-			return nil, err
+			// when the JSON can't be parsed, data was probably empty or a plain string,
+			// so we try to return a helpful error anyway
+			return nil, fmt.Errorf("Unknown API Error: %d %s", resp.StatusCode, string(data))
 		}
 		return nil, errors.New(errMap["message"].(string))
 	}