Fix for #1564: Return empty array instead of single element nil array in value array processor (#1993)

This changes the return of a nil result through the `RedisValueArrayProcessor` so that it's a `[]` instead of a single element `[ nil ]` in our handling.

This affects the following commands, which are multibulk when a count is provided (even if it's 1), otherwise they are bulkstring. This change affects the non-count case when it's null. Instead of a single element array with a nil value, we'd return an empty array as the Redis surface area intends:
- `LPOP`/`RPOP`
- `SRANDMEMBER`
- `SPOP`

The other usages of `RedisValueArrayProcessor` are _always_ multibulk and are not affected:
- `HMGET`
- `HKEYS`
- `HVALS`
- `LRANGE`
- `MGET`
- `SDIFF`
- `SINTER`
- `SUNION`
- `SMEMBERS`
- `SORT`
- `XCLAIM`
- `Z(REV)RANGE`
- `Z(REV)RANGEBYLEX`
- `Z(REV)RANGEBYSCORE`

Author: NickCraver

docs/ReleaseNotes.md

@@ -16,6 +16,7 @@
   - More correctly reconnects subscriptions on connection failures, including to other endpoints
 - Adds "(vX.X.X)" version suffix to the default client ID so server-side `CLIENT LIST` can more easily see what's connected (#1985 via NickCraver)
 - Fix for including (or not including) key names on some message failures (#1990 via NickCraver)
+- Fixed return of nil results in `LPOP`, `RPOP`, `SRANDMEMBER`, and `SPOP` (#1993 via NickCraver)
 
 ## 2.2.88
 

src/StackExchange.Redis/ResultProcessor.cs

@@ -1249,7 +1249,10 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                 {
                     // allow a single item to pass explicitly pretending to be an array; example: SPOP {key} 1
                     case ResultType.BulkString:
-                        var arr = new[] { result.AsRedisValue() };
+                        // If the result is nil, the result should be an empty array
+                        var arr = result.IsNull
+                            ? Array.Empty<RedisValue>()
+                            : new[] { result.AsRedisValue() };
                         SetResult(message, arr);
                         return true;
                     case ResultType.MultiBulk:

tests/StackExchange.Redis.Tests/Sets.cs

@@ -84,6 +84,7 @@ public void SetPopMulti_Multi()
                 Assert.Equal(7, db.SetLength(key));
             }
         }
+
         [Fact]
         public void SetPopMulti_Single()
         {
@@ -225,5 +226,22 @@ public async Task SetAdd_Zero_Async()
                 Assert.Equal(0, db.SetLength(key));
             }
         }
+
+        [Fact]
+        public void SetPopMulti_Nil()
+        {
+            using (var conn = Create())
+            {
+                Skip.IfMissingFeature(conn, nameof(RedisFeatures.SetPopMultiple), r => r.SetPopMultiple);
+
+                var db = conn.GetDatabase();
+                var key = Me();
+
+                db.KeyDelete(key, CommandFlags.FireAndForget);
+
+                var arr = db.SetPop(key, 1);
+                Assert.Empty(arr);
+            }
+        }
     }
 }