[modular] Make user_controller optional in UserLoginParams.

This will be removed in subsequent CL.

TESTED: built with topaz CL
Change-Id: I4798849af3aa2b37def5542ac9f9a05a301c14c6
diff --git a/sdk/fidl/fuchsia.modular/basemgr/user_provider.fidl b/sdk/fidl/fuchsia.modular/basemgr/user_provider.fidl
index c90aa7b..c10c69a 100644
--- a/sdk/fidl/fuchsia.modular/basemgr/user_provider.fidl
+++ b/sdk/fidl/fuchsia.modular/basemgr/user_provider.fidl
@@ -59,7 +59,7 @@
     fuchsia.sys.ServiceProvider? services;
 
     // If login fails, |user_controller| is closed.
-    request<UserController> user_controller;
+    request<UserController>? user_controller;
 };
 
 // Used to specify arguments to log into a user session.
@@ -77,7 +77,7 @@
     fuchsia.sys.ServiceProvider? services;
 
     // If login fails, |user_controller| is closed.
-    request<UserController> user_controller;
+    request<UserController>? user_controller;
 };
 
 // Provided by the |Basemgr| to the |BaseShell| when it authenticates a