AddModuleAsync/AddModulesAsync should not require an IServiceProvider

If one is not passed, they will default to an EmptyServiceProvider

This resolves issues where since the merging of OnModuleBuilding, users
were effectively forced to use the DI pattern, where before they were
not.

While this isn't necessarily a bad idea, we shouldn't just change this
behavior for no reason (though I assume making the IServiceProvider
argument required was a mistake)

src/Discord.Net.Commands/CommandService.cs

@@ -95,9 +95,11 @@ namespace Discord.Commands
                 _moduleLock.Release();
             }
         }
-        public Task<ModuleInfo> AddModuleAsync<T>(IServiceProvider services) => AddModuleAsync(typeof(T), services);
-        public async Task<ModuleInfo> AddModuleAsync(Type type, IServiceProvider services)
+        public Task<ModuleInfo> AddModuleAsync<T>(IServiceProvider services = null) => AddModuleAsync(typeof(T), services);
+        public async Task<ModuleInfo> AddModuleAsync(Type type, IServiceProvider services = null)
         {
+            services = services ?? EmptyServiceProvider.Instance;
+
             await _moduleLock.WaitAsync().ConfigureAwait(false);
             try
             {
@@ -120,8 +122,10 @@ namespace Discord.Commands
                 _moduleLock.Release();
             }
         }
-        public async Task<IEnumerable<ModuleInfo>> AddModulesAsync(Assembly assembly, IServiceProvider services)
+        public async Task<IEnumerable<ModuleInfo>> AddModulesAsync(Assembly assembly, IServiceProvider services = null)
         {
+            services = services ?? EmptyServiceProvider.Instance;
+
             await _moduleLock.WaitAsync().ConfigureAwait(false);
             try
             {

src/Discord.Net.Commands/Utilities/ReflectionUtils.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Reflection;