[geary] Fix critical error when the draft manager reports a null draft id.

[Michael Gratton <mjog src gnome org>]

commit adcd62de21cf11a560619a2748d0b2f2ed1b6448
Author: Michael James Gratton <mike vee net>
Date:   Sun Oct 16 23:44:15 2016 +1100

    Fix critical error when the draft manager reports a null draft id.
    
    * src/client/composer/composer-widget.vala
      (ComposerWidget::draft_id_changed): Make the id param nullable.

 src/client/composer/composer-widget.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/src/client/composer/composer-widget.vala b/src/client/composer/composer-widget.vala
index 188b425..6a13215 100644
--- a/src/client/composer/composer-widget.vala
+++ b/src/client/composer/composer-widget.vala
@@ -378,7 +378,7 @@ public class ComposerWidget : Gtk.EventBox {
     }
 
 
-    public signal void draft_id_changed(Geary.EmailIdentifier id);
+    public signal void draft_id_changed(Geary.EmailIdentifier? id);
 
 
     public ComposerWidget(Geary.Account account, ComposeType compose_type,