Avoid formatter lookup for empty array

Also revert a change made in this PR that is irrelevant and not consistent with typical style.

Author: AArnott

src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Formatters/CollectionFormatter.cs

@@ -793,14 +793,14 @@ public IList Deserialize(ref MessagePackReader reader, MessagePackSerializerOpti
                 return default(IList);
             }
 
-            IMessagePackFormatter<object> formatter = options.Resolver.GetFormatterWithVerify<object>();
-
             var count = reader.ReadArrayHeader();
             if (count == 0)
             {
                 return Array.Empty<object>();
             }
 
+            IMessagePackFormatter<object> formatter = options.Resolver.GetFormatterWithVerify<object>();
+
             var list = new object[count];
             options.Security.DepthStep(ref reader);
             try

src/MessagePack.UnityClient/Assets/Scripts/MessagePack/Formatters/DateTimeFormatters.cs

@@ -51,7 +51,7 @@ public DateTime[] Deserialize(ref MessagePackReader reader, MessagePackSerialize
         {
             if (reader.TryReadNil())
             {
-                return default;
+                return null;
             }
 
             var len = reader.ReadArrayHeader();