Update sort_ro reply_schema to mention the null reply (#12534)

Also added a test to cover this case, so this can
cover the reply schemas check.

(cherry picked from commit 9ce8c54d74f40bc5a70f210c2b14cf89c031ee10)

src/commands/sort.json

@@ -150,7 +150,7 @@
                                 "type": "string"
                             },
                             {
-                                "description": "GET option is specified, but no object was found ",
+                                "description": "GET option is specified, but no object was found",
                                 "type": "null"
                             }
                         ]

src/commands/sort_ro.json

@@ -117,7 +117,15 @@
             "description": "a list of sorted elements",
             "type": "array",
             "items": {
-                "type": "string"
+                "oneOf": [
+                    {
+                        "type": "string"
+                    },
+                    {
+                        "description": "GET option is specified, but no object was found",
+                        "type": "null"
+                    }
+                ]
             }
         }
     }

tests/unit/sort.tcl

@@ -75,12 +75,14 @@ start_server {
         assert_equal [lsort -integer $result] [r sort tosort GET #]
     } {} {cluster:skip}
 
-    test "SORT GET <const>" {
+foreach command {SORT SORT_RO} {
+    test "$command GET <const>" {
         r del foo
-        set res [r sort tosort GET foo]
+        set res [r $command tosort GET foo]
         assert_equal 16 [llength $res]
         foreach item $res { assert_equal {} $item }
     } {} {cluster:skip}
+}
 
     test "SORT GET (key and hash) with sanity check" {
         set l1 [r sort tosort GET # GET weight_*]