Fix SPOP with count returning nil instead of an empty set for a missing key

libs/server/Resp/Objects/SetCommands.cs

@@ -535,8 +535,7 @@ private unsafe bool SetPop<TGarnetApi>(ref TGarnetApi storageApi)
 
                 if (countParameter == 0)
                 {
-                    while (!RespWriteUtils.TryWriteEmptyArray(ref dcurr, dend))
-                        SendAndReset();
+                    WriteEmptySet();
 
                     return true;
                 }
@@ -558,6 +557,13 @@ private unsafe bool SetPop<TGarnetApi>(ref TGarnetApi storageApi)
                     ProcessOutput(output.SpanByteAndMemory);
                     break;
                 case GarnetStatus.NOTFOUND:
+                    if (parseState.Count == 2)
+                    {
+                        // SPOP key count replies with an empty set when the key does not exist
+                        WriteEmptySet();
+                        break;
+                    }
+
                     WriteNull();
                     break;
                 case GarnetStatus.WRONGTYPE:

test/standalone/Garnet.test.collections/RespSetTest.cs

@@ -1483,6 +1483,15 @@ public void CanDoSPOPCommandWhenKeyDoesNotExistLC()
             TestUtils.AssertEqualUpToExpectedLength(expectedResponse, response);
         }
 
+        [Test]
+        public void CanDoSPOPWithCountCommandWhenKeyDoesNotExistLC()
+        {
+            using var lightClientRequest = TestUtils.CreateRequest();
+            var response = lightClientRequest.SendCommand("SPOP fooset 3");
+            var expectedResponse = "*0\r\n";
+            TestUtils.AssertEqualUpToExpectedLength(expectedResponse, response);
+        }
+
         [Test]
         public void CanUseNotExistingSetwithSMembers()
         {