ghub-{host,username}: Handle callers passing nil for FORGE or HOST

They shouldn't do that, but the current version of Forge still does.

Author: tarsius

lisp/ghub.el

@@ -756,12 +756,15 @@ or (info \"(ghub)Getting Started\") for instructions."
              host))
     (if (functionp token) (funcall token) token)))
 
-(cl-defmethod ghub--host (&optional (forge 'github))
-  (or (ghub--git-get (format "%s.host" forge))
-      (alist-get forge ghub-default-host-alist)))
-
-(cl-defmethod ghub--username (host &optional (forge 'github))
-  (let ((var (format "%s.%s.user" forge host)))
+(cl-defmethod ghub--host (&optional forge)
+  (let ((forge (or forge 'github)))
+    (or (ghub--git-get (format "%s.host" forge))
+        (alist-get forge ghub-default-host-alist))))
+
+(cl-defmethod ghub--username (host &optional forge)
+  (let* ((forge (or forge 'github))
+         (host (or host (ghub--host forge)))
+         (var (format "%s.%s.user" forge host)))
     (or (ghub--git-get var)
         (if-let (((equal host (alist-get forge ghub-default-host-alist)))
                  (default-var (format "%s.user" forge)))