Don't popup context menu with no items, fixes #2032.

On OS X popuping an empty context menu would have no effect but on Linux
an empty menu container would still be showed.

src/context-menu-manager.coffee

@@ -98,5 +98,6 @@ class ContextMenuManager
   showForEvent: (event) ->
     @activeElement = event.target
     menuTemplate = @combinedMenuTemplateForElement(event.target)
+    return unless menuTemplate?.length > 0
     @executeBuildHandlers(event, menuTemplate)
     remote.getCurrentWindow().emit('context-menu', menuTemplate)