Fix build errors

Visual Studio didn't build it properly, and showed that everything was
ok. Whoops.

src/Discord.Net.Commands/CommandMatch.cs

@@ -16,7 +16,7 @@ namespace Discord.Commands
 
         public Task<PreconditionResult> CheckPreconditionsAsync(ICommandContext context, IDependencyMap map = null)
             => Command.CheckPreconditionsAsync(context, map);
-        public Task<ParseResult> ParseAsync(ICommandContext context, SearchResult searchResult, PreconditionResult? preconditionResult = null)
+        public Task<ParseResult> ParseAsync(ICommandContext context, SearchResult searchResult, PreconditionResult preconditionResult = null)
             => Command.ParseAsync(context, Alias.Length, searchResult, preconditionResult);
         public Task<ExecuteResult> ExecuteAsync(ICommandContext context, IEnumerable<object> argList, IEnumerable<object> paramList, IDependencyMap map)
             => Command.ExecuteAsync(context, argList, paramList, map);

src/Discord.Net.Commands/Info/CommandInfo.cs

@@ -85,12 +85,12 @@ namespace Discord.Commands
             return PreconditionResult.FromSuccess();
         }
         
-        public async Task<ParseResult> ParseAsync(ICommandContext context, int startIndex, SearchResult searchResult, PreconditionResult? preconditionResult = null)
+        public async Task<ParseResult> ParseAsync(ICommandContext context, int startIndex, SearchResult searchResult, PreconditionResult preconditionResult = null)
         {
             if (!searchResult.IsSuccess)
                 return ParseResult.FromError(searchResult);
-            if (preconditionResult != null && !preconditionResult.Value.IsSuccess)
-                return ParseResult.FromError(preconditionResult.Value);
+            if (preconditionResult != null && !preconditionResult.IsSuccess)
+                return ParseResult.FromError(preconditionResult);
             
             string input = searchResult.Text.Substring(startIndex);
             return await CommandParser.ParseArgs(this, context, input, 0).ConfigureAwait(false);