naughty: Fix builds without dbus

The new notification action code tried to emit dbus signals even if awesome was
build without notification support. Fix this by adding the necessary "if".

(This also removes an unused return value which wasn't marked as "local" and
thus triggered my "complain if something messes with the global env"-script)

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/naughty.lua.in

@@ -259,17 +259,21 @@ local function getById(id)
 end
 
 local function sendActionInvoked(notificationId, action)
-    res = capi.dbus.emit_signal("session", "/org/freedesktop/Notifications",
+    if capi.dbus then
+        capi.dbus.emit_signal("session", "/org/freedesktop/Notifications",
         "org.freedesktop.Notifications", "ActionInvoked",
         "i", notificationId,
         "s", action)
+    end
 end
 
 local function sendNotificationClosed(notificationId, reason)
-    res = capi.dbus.emit_signal("session", "/org/freedesktop/Notifications",
+    if capi.dbus then
+        capi.dbus.emit_signal("session", "/org/freedesktop/Notifications",
         "org.freedesktop.Notifications", "NotificationClosed",
         "i", notificationId,
         "i", reason)
+    end
 end
 
 --- Create notification. args is a dictionary of (optional) arguments.