[gitg/ricotz/vala] Properly mark "head" parameter of ActionSupport.stash_if_needed() as nullable

[Rico Tzschichholz <ricotz src gnome org>]

commit f507f7487c831fee20ec6c196d6f467a392b29dd
Author: Rico Tzschichholz <ricotz ubuntu com>
Date:   Mon Feb 22 14:59:27 2021 +0100

    Properly mark "head" parameter of ActionSupport.stash_if_needed() as nullable
    
    Fixes gitg-FATAL-CRITICAL:
        gitg_action_support_stash_if_needed: assertion 'head != NULL' failed

 gitg/gitg-action-support.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gitg/gitg-action-support.vala b/gitg/gitg-action-support.vala
index 6eb7ad8d..d864fffc 100644
--- a/gitg/gitg-action-support.vala
+++ b/gitg/gitg-action-support.vala
@@ -122,7 +122,7 @@ public class ActionSupport : Object
                return head != null;
        }
 
-       public async bool stash_if_needed(SimpleNotification notification, Gitg.Ref head)
+       public async bool stash_if_needed(SimpleNotification notification, Gitg.Ref? head)
        {
                // Offer to stash if there are any local changes
                if ((yield working_directory_dirty()))