Replaced fmt.Println statements with log.Println.

Return "empty command" error from handleCommand method if an empty command is passed to the server.
Wait until connection is no longer nil in acl package tests.

internal/modules/sorted_set/commands_test.go

@@ -1589,7 +1589,7 @@ func Test_HandleZMPOP(t *testing.T) {
 			preset: true,
 			presetValues: map[string]interface{}{
 				"ZmpopKey8": "Default value",
-				"ZmpopKey9": 56,
+				"ZmpopKey9": "56",
 				"ZmpopKey11": sorted_set.NewSortedSet([]sorted_set.MemberParam{
 					{Value: "one", Score: 1}, {Value: "two", Score: 2},
 					{Value: "three", Score: 3}, {Value: "four", Score: 4},