[geary/gnumdk/vala_nullable_enum_property] engine: Vala disallows using nullabe enum value as real GObject property

[Cédric Bellegarde <cbellegarde src gnome org>]

commit a2e59b92a396e5be1954a26bf16d22f4231bbb40
Author: Cédric Bellegarde <cedric bellegarde adishatz org>
Date:   Wed Jul 20 14:35:30 2022 +0200

    engine: Vala disallows using nullabe enum value as real GObject property

 src/engine/imap/api/imap-client-service.vala       | 2 +-
 src/engine/imap/api/imap-session-object.vala       | 9 +++++----
 src/engine/imap/transport/imap-client-session.vala | 3 ++-
 3 files changed, 8 insertions(+), 6 deletions(-)
---
diff --git a/src/engine/imap/api/imap-client-service.vala b/src/engine/imap/api/imap-client-service.vala
index da3598f38..fe534b498 100644
--- a/src/engine/imap/api/imap-client-service.vala
+++ b/src/engine/imap/api/imap-client-service.vala
@@ -558,7 +558,7 @@ public class Geary.Imap.ClientService : Geary.ClientService {
         var session = source as ClientSession;
         if (session != null &&
             session.protocol_state == NOT_CONNECTED &&
-            session.disconnected != null) {
+            session.disconnected != ClientSession.DisconnectReason.NULL) {
             debug(
                 "Session disconnected: %s: %s",
                 session.to_string(),
diff --git a/src/engine/imap/api/imap-session-object.vala b/src/engine/imap/api/imap-session-object.vala
index 80695fcaf..24fbf5e67 100644
--- a/src/engine/imap/api/imap-session-object.vala
+++ b/src/engine/imap/api/imap-session-object.vala
@@ -109,10 +109,11 @@ public abstract class Geary.Imap.SessionObject : BaseObject, Logging.Source {
             // Disconnect reason will null when the session is being
             // logged out but the logout command has not yet been
             // completed.
-            var reason = (
-                this.session.disconnected ??
-                ClientSession.DisconnectReason.LOCAL_CLOSE
-            );
+            var reason =
+                this.session.disconnected ==
+                    ClientSession.DisconnectReason.NULL ?
+                ClientSession.DisconnectReason.LOCAL_CLOSE :
+                this.session.disconnected;
             close();
             disconnected(reason);
         }
diff --git a/src/engine/imap/transport/imap-client-session.vala 
b/src/engine/imap/transport/imap-client-session.vala
index 9f1eb94e8..31aa16afc 100644
--- a/src/engine/imap/transport/imap-client-session.vala
+++ b/src/engine/imap/transport/imap-client-session.vala
@@ -108,6 +108,7 @@ public class Geary.Imap.ClientSession : BaseObject, Logging.Source {
     }
 
     public enum DisconnectReason {
+        NULL,
         LOCAL_CLOSE,
         LOCAL_ERROR,
         REMOTE_CLOSE,
@@ -277,7 +278,7 @@ public class Geary.Imap.ClientSession : BaseObject, Logging.Source {
     }
 
     /** Specifies the reason the session was disconnected, if any. */
-    public DisconnectReason? disconnected { get; private set; default = null; }
+    public DisconnectReason disconnected { get; private set; default = NULL; }
 
     /**
      * Set of IMAP extensions reported as being supported by the server.