fix: Added Msg.Content Null Check For Prefixes (#1200)

* Added Msg.Content Null Check

* Minor Change

* Grouped Params In If Statement

* Minor Change

src/Discord.Net.Commands/Extensions/MessageExtensions.cs

@@ -19,7 +19,7 @@ namespace Discord.Commands
         public static bool HasCharPrefix(this IUserMessage msg, char c, ref int argPos)
         {
             var text = msg.Content;
-            if (text.Length > 0 && text[0] == c)
+            if (!string.IsNullOrEmpty(text) && text[0] == c)
             {
                 argPos = 1;
                 return true;
@@ -32,7 +32,7 @@ namespace Discord.Commands
         public static bool HasStringPrefix(this IUserMessage msg, string str, ref int argPos, StringComparison comparisonType = StringComparison.Ordinal)
         {
             var text = msg.Content;
-            if (text.StartsWith(str, comparisonType))
+            if (!string.IsNullOrEmpty(text) && text.StartsWith(str, comparisonType))
             {
                 argPos = str.Length;
                 return true;
@@ -45,7 +45,7 @@ namespace Discord.Commands
         public static bool HasMentionPrefix(this IUserMessage msg, IUser user, ref int argPos)
         {
             var text = msg.Content;
-            if (text.Length <= 3 || text[0] != '<' || text[1] != '@') return false;
+            if (string.IsNullOrEmpty(text) || text.Length <= 3 || text[0] != '<' || text[1] != '@') return false;
 
             int endPos = text.IndexOf('>');
             if (endPos == -1) return false;