Fix Issue #446. RestUserMessage initializes _reactions even if there are no reactions so get doesn't cause exception.

src/Discord.Net.Rest/Entities/Messages/RestUserMessage.cs

@@ -115,6 +115,8 @@ namespace Discord.Rest
                 else
                     _reactions = ImmutableArray.Create<RestReaction>();
             }
+            else
+                _reactions = ImmutableArray.Create<RestReaction>();
 
             if (model.Content.IsSpecified)
             {