fix: Possible NRE in Sanitize (#2290)

src/Discord.Net.Core/Format.cs

@@ -37,6 +37,7 @@ namespace Discord
         /// <summary> Sanitizes the string, safely escaping any Markdown sequences. </summary>
         public static string Sanitize(string text)
         {
+            if (text != null)
                 foreach (string unsafeChar in SensitiveCharacters)
                     text = text.Replace(unsafeChar, $"\\{unsafeChar}");
             return text;