Allow empty assignees on pull request edit (#22150) (#22214)

Backport of #22150

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

routers/api/v1/repo/pull.go

@@ -491,6 +491,11 @@ func EditPullRequest(ctx *context.APIContext) {
 	issue := pr.Issue
 	issue.Repo = ctx.Repo.Repository
 
+	if err := issue.LoadAttributes(ctx); err != nil {
+		ctx.Error(http.StatusInternalServerError, "LoadAttributes", err)
+		return
+	}
+
 	if !issue.IsPoster(ctx.Doer.ID) && !ctx.Repo.CanWrite(unit.TypePullRequests) {
 		ctx.Status(http.StatusForbidden)
 		return