[geary/mjog/vala-async-param-assert-fixes] Geary.RFC822.Message: Fix some missing nullable annotations

[Michael Gratton <mjog src gnome org>]

commit ea8778cf623aa0fbdf5fae3f302be91c80af538a
Author: Michael Gratton <mike vee net>
Date:   Thu Jun 25 10:32:02 2020 +1000

    Geary.RFC822.Message: Fix some missing nullable annotations
    
    Now that vala is assert-checking params on async methods
    (GNOME/vala#1005), fix a few places where they turn up problems in the
    engine.

 src/engine/rfc822/rfc822-message.vala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/engine/rfc822/rfc822-message.vala b/src/engine/rfc822/rfc822-message.vala
index cbdb7dce..476a3b40 100644
--- a/src/engine/rfc822/rfc822-message.vala
+++ b/src/engine/rfc822/rfc822-message.vala
@@ -519,7 +519,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
     private async GMime.Part? get_buffer_part(Memory.Buffer buffer,
                                               string basename,
                                               Geary.Mime.DispositionType disposition,
-                                              GLib.Cancellable cancellable)
+                                              GLib.Cancellable? cancellable)
         throws GLib.Error {
         Mime.ContentType? mime_type = Mime.ContentType.guess_type(
             basename,
@@ -559,7 +559,7 @@ public class Geary.RFC822.Message : BaseObject, EmailHeaderSet {
     private async GMime.Part finalise_attachment_part(GMime.Stream stream,
                                                       GMime.Part part,
                                                       GMime.ContentType content_type,
-                                                      GLib.Cancellable cancellable)
+                                                      GLib.Cancellable? cancellable)
         throws GLib.Error {
 
         // Text parts should be scanned fully to determine best