[geary] Geary.RFC822.Message: Fix criticals from missing parameter nullable annotation

[Michael Gratton <mjog src gnome org>]

commit 03af13cbf1cedccf9075d5088d39093a432c34de
Author: Michael James Gratton <mike vee net>
Date:   Thu Jul 2 11:05:55 2020 +1000

    Geary.RFC822.Message: Fix criticals from missing parameter nullable annotation

 src/engine/rfc822/rfc822-message.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/engine/rfc822/rfc822-message.vala b/src/engine/rfc822/rfc822-message.vala
index 476a3b408..57c587351 100644
--- a/src/engine/rfc822/rfc822-message.vala
+++ b/src/engine/rfc822/rfc822-message.vala
@@ -490,7 +490,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
 
     private async GMime.Part? get_file_part(File file,
                                             Geary.Mime.DispositionType disposition,
-                                            GLib.Cancellable cancellable)
+                                            GLib.Cancellable? cancellable)
         throws GLib.Error {
         FileInfo file_info = yield file.query_info_async(
             FileAttribute.STANDARD_CONTENT_TYPE,