sysupdate: include default component for feature-all

--component-all is documented to include the default component-less
installation. Do not drop it merely because the context operates on a
root/image, or because all its transfers are currently disabled by
features.

This lets --component-all --feature-all enable-feature write the
default component feature drop-ins instead of succeeding with no
components selected.

TEST-72-SYSUPDATE covers both all transfers disabled by features and
the same default component feature operation under --root=.

Repro: create a default feata.feature plus a transfer gated by feata,
then run:
build/systemd-sysupdate --root="$root" --component-all --feature-all enable-feature

Before: no drop-in was written.

Follow-up for: 4481661a75

src/sysupdate/sysupdate.c

@@ -2949,15 +2949,13 @@ static int context_list_components(Context *context, char ***ret_component_names
         if (ret_component_names)
                 *ret_component_names = TAKE_PTR(z);
 
-        /* Does the system have at least one transfer file in /etc/sysupdate.d, which can be considered a
-         * TARGET_HOST? See target_get_argument() in sysupdated.c */
+        /* Does the system have at least one transfer file in sysupdate.d, which can be considered the
+         * default component-less installation? */
         if (ret_has_default_component)
                 *ret_has_default_component =
                         !context->definitions &&
                         !context->component &&
-                        !context->root &&
-                        !context->image &&
-                        context->n_transfers > 0;
+                        context->n_transfers + context->n_disabled_transfers > 0;
 
         return 0;
 }