Fix client to return error when nil task is passed

client_test.go

@@ -857,6 +857,11 @@ func TestClientEnqueueError(t *testing.T) {
 		task *Task
 		opts []Option
 	}{
+		{
+			desc: "With nil task",
+			task: nil,
+			opts: []Option{},
+		},
 		{
 			desc: "With empty queue name",
 			task: task,