Check whether user is nil in Prepare() of users API (#7507)
Check whether user is nil in Prepare() of users API (#7507)

Signed-off-by: Daniel Jiang <b8e1ca884a84f218767196db52915b5edb9f310e@vmware.com>
wy65701436/harbor,wy65701436/harbor,reasonerjt/harbor,reasonerjt/harbor,wy65701436/harbor,steven-zou/harbor,reasonerjt/harbor,steven-zou/harbor,steven-zou/harbor,wy65701436/harbor,wy65701436/harbor,wy65701436/harbor,reasonerjt/harbor,reasonerjt/harbor,reasonerjt/harbor,steven-zou/harbor,steven-zou/harbor,steven-zou/harbor