Fixes LPOP/RPOP wrong replies when count is 0 (#9692)

Introduced in #8179, this fixes the command's replies in the 0 count edge case.
[BREAKING] changes the reply type when count is 0 to an empty array (instead of nil)
Moves LPOP ... 0 fast exit path after type check to reply with WRONGTYPE

src/t_list.c

@@ -503,6 +503,7 @@ void listElementsRemoved(client *c, robj *key, int where, robj *o, long count, i
  * optional count may be provided as the third argument of the client's
  * command. */
 void popGenericCommand(client *c, int where) {
+    int hascount = (c->argc == 3);
     long count = 0;
     robj *value;
 
@@ -510,21 +511,22 @@ void popGenericCommand(client *c, int where) {
         addReplyErrorFormat(c,"wrong number of arguments for '%s' command",
                             c->cmd->name);
         return;
-    } else if (c->argc == 3) {
+    } else if (hascount) {
         /* Parse the optional count argument. */
         if (getPositiveLongFromObjectOrReply(c,c->argv[2],&count,NULL) != C_OK) 
             return;
-        if (count == 0) {
-            /* Fast exit path. */
-            addReplyNullArray(c);
-            return;
-        }
     }
 
     robj *o = lookupKeyWriteOrReply(c, c->argv[1], shared.null[c->resp]);
     if (o == NULL || checkType(c, o, OBJ_LIST))
         return;
 
+    if (hascount && !count) {
+        /* Fast exit path. */
+        addReply(c,shared.emptyarray);
+        return;
+    }
+
     if (!count) {
         /* Pop a single element. This is POP's original behavior that replies
          * with a bulk string. */

tests/unit/type/list.tcl

@@ -517,10 +517,9 @@ start_server {
         assert_equal $largevalue(linkedlist) [r rpop mylist2]
         assert_equal c [r lpop mylist2]
     }
-    
-    test {R/LPOP with the optional count argument} {
+
+    test {RPOP/LPOP with the optional count argument} {
         assert_equal 7 [r lpush listcount aa bb cc dd ee ff gg]
-        assert_equal {} [r lpop listcount 0]
         assert_equal {gg} [r lpop listcount 1]
         assert_equal {ff ee} [r lpop listcount 2]
         assert_equal {aa bb} [r rpop listcount 2]
@@ -529,6 +528,16 @@ start_server {
         assert_error "*ERR*range*" {r lpop forbarqaz -123}
     }
 
+    # Make sure we can distinguish between an empty array and a null response
+    r readraw 1
+
+    test {RPOP/LPOP with the count 0 returns an empty array} {
+        r lpush listcount zero
+        r lpop listcount 0
+    } {*0}
+
+    r readraw 0
+
     test {Variadic RPUSH/LPUSH} {
         r del mylist
         assert_equal 4 [r lpush mylist a b c d]