Fix API Bug (fail on empty assignees) (#8873)

* keep sure if assigneeIDs == nil -> do nothing

* fix #8872

* Revert "keep sure if assigneeIDs == nil -> do nothing"

-> go handle it itself preaty well

This reverts commit e72d94129c.

* clarity comparson

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* simplify

* Update models/issue_assignees.go

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* Update issue_assignees.go

* simplify more

* add --if oneAssignee != ""-- again

* Update models/issue_assignees.go

Co-Authored-By: guillep2k <18600385+guillep2k@users.noreply.github.com>

* CI.restart()

* Update issue_assignees.go

* add Test for GetUserIDsByNames

* add Test for MakeIDsFromAPIAssigneesToAdd

* fix test

models/issue_assignees.go

@@ -7,6 +7,8 @@ package models
 import (
 	"fmt"
 
+	"code.gitea.io/gitea/modules/util"
+
 	"xorm.io/xorm"
 )
 
@@ -171,25 +173,20 @@ func toggleUserAssignee(e *xorm.Session, issue *Issue, assigneeID int64) (remove
 // MakeIDsFromAPIAssigneesToAdd returns an array with all assignee IDs
 func MakeIDsFromAPIAssigneesToAdd(oneAssignee string, multipleAssignees []string) (assigneeIDs []int64, err error) {
 
+	var requestAssignees []string
+
 	// Keeping the old assigning method for compatibility reasons
-	if oneAssignee != "" {
+	if oneAssignee != "" && !util.IsStringInSlice(oneAssignee, multipleAssignees) {
+		requestAssignees = append(requestAssignees, oneAssignee)
+	}
 
-		// Prevent double adding assignees
-		var isDouble bool
-		for _, assignee := range multipleAssignees {
-			if assignee == oneAssignee {
-				isDouble = true
-				break
-			}
-		}
-
-		if !isDouble {
-			multipleAssignees = append(multipleAssignees, oneAssignee)
-		}
+	//Prevent empty assignees
+	if len(multipleAssignees) > 0 && multipleAssignees[0] != "" {
+		requestAssignees = append(requestAssignees, multipleAssignees...)
 	}
 
 	// Get the IDs of all assignees
-	assigneeIDs, err = GetUserIDsByNames(multipleAssignees, false)
+	assigneeIDs, err = GetUserIDsByNames(requestAssignees, false)
 
 	return
 }