Fixed ImapUtils.ParseContentType to handle a null type

Author: jstedfast

MailKit/Net/Imap/ImapUtils.cs

@@ -1151,6 +1151,7 @@ static ContentType ParseContentType (ImapEngine engine, string format, Cancellat
 					}
 				}
 			} else {
+				type ??= "application";
 				subtype = ReadStringToken (engine, format, cancellationToken);
 			}
 
@@ -1207,6 +1208,7 @@ static async Task<ContentType> ParseContentTypeAsync (ImapEngine engine, string
 					}
 				}
 			} else {
+				type ??= "application";
 				subtype = await ReadStringTokenAsync (engine, format, cancellationToken).ConfigureAwait (false);
 			}