Default DependencyMap to an empty map when not supplied

src/Discord.Net.Commands/CommandInfo.cs

@@ -87,6 +87,9 @@ namespace Discord.Commands
 
         public async Task<PreconditionResult> CheckPreconditions(CommandContext context, IDependencyMap map = null)
         {
+            if (map == null)
+                map = DependencyMap.Empty;
+
             foreach (PreconditionAttribute precondition in Module.Preconditions)
             {
                 var result = await precondition.CheckPermissions(context, this, map).ConfigureAwait(false);
@@ -150,6 +153,9 @@ namespace Discord.Commands
         }
         public async Task<ExecuteResult> Execute(CommandContext context, IEnumerable<object> argList, IEnumerable<object> paramList, IDependencyMap map)
         {
+            if (map == null)
+                map = DependencyMap.Empty;
+
             try
             {
                 var args = GenerateArgs(argList, paramList);

src/Discord.Net.Commands/CommandService.cs

@@ -195,6 +195,8 @@ namespace Discord.Commands
             => Execute(context, context.Message.Content.Substring(argPos), dependencyMap, multiMatchHandling);
         public async Task<IResult> Execute(CommandContext context, string input, IDependencyMap dependencyMap = null, MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)
         {
+            dependencyMap = dependencyMap ?? DependencyMap.Empty;
+
             var searchResult = Search(context, input);
             if (!searchResult.IsSuccess)
                 return searchResult;

src/Discord.Net.Commands/Dependencies/DependencyMap.cs

@@ -7,6 +7,8 @@ namespace Discord.Commands
     {
         private Dictionary<Type, object> map;
 
+        public static DependencyMap Empty => new DependencyMap();
+
         public DependencyMap()
         {
             map = new Dictionary<Type, object>();