Fixed TextPreviewer to use an encoding with an empty string fallback

This was causing some MailKit unit tests to fail because the default
for Encoding.GetEncoding(int codepage) must have changed between net6.0
and net8.0 to fall back to using "?" as a fallback string for incomplete
or invalud character sequences.

Anyway, explicitly use string.Empty as the fallback.

MimeKit/Text/TextPreviewer.cs

@@ -189,7 +189,7 @@ public virtual string GetPreviewText (Stream stream, string charset)
 			Encoding encoding;
 
 			try {
-				encoding = CharsetUtils.GetEncoding (charset);
+				encoding = CharsetUtils.GetEncoding (charset, string.Empty);
 			} catch (NotSupportedException) {
 				encoding = CharsetUtils.UTF8;
 			}