Fixed ModifyAsync null checks

src/Discord.Net/Entities/Channels/GuildChannel.cs

@@ -51,7 +51,7 @@ namespace Discord
         }
         public async Task ModifyAsync(Action<ModifyGuildChannelParams> func)
         {
-            if (func != null) throw new NullReferenceException(nameof(func));
+            if (func == null) throw new NullReferenceException(nameof(func));
 
             var args = new ModifyGuildChannelParams();
             func(args);

src/Discord.Net/Entities/Channels/TextChannel.cs

@@ -32,7 +32,7 @@ namespace Discord
 
         public async Task ModifyAsync(Action<ModifyTextChannelParams> func)
         {
-            if (func != null) throw new NullReferenceException(nameof(func));
+            if (func == null) throw new NullReferenceException(nameof(func));
 
             var args = new ModifyTextChannelParams();
             func(args);

src/Discord.Net/Entities/Channels/VoiceChannel.cs

@@ -28,7 +28,7 @@ namespace Discord
         
         public async Task ModifyAsync(Action<ModifyVoiceChannelParams> func)
         {
-            if (func != null) throw new NullReferenceException(nameof(func));
+            if (func == null) throw new NullReferenceException(nameof(func));
 
             var args = new ModifyVoiceChannelParams();
             func(args);

src/Discord.Net/Entities/Users/SelfUser.cs

@@ -33,7 +33,7 @@ namespace Discord
         }
         public async Task ModifyAsync(Action<ModifyCurrentUserParams> func)
         {
-            if (func != null) throw new NullReferenceException(nameof(func));
+            if (func == null) throw new NullReferenceException(nameof(func));
 
             var args = new ModifyCurrentUserParams();
             func(args);