Files.app: Make empty folder background hidden at first.

BUG=475313
TEST=manually

Review URL: https://codereview.chromium.org/1077513002

Cr-Commit-Position: refs/heads/master@{#324391}

ui/file_manager/file_manager/main.html

@@ -462,7 +462,7 @@
                 Welcome to the new epic photo importer!
               </div>
               <div id="list-container">
-                <div id="empty-folder">
+                <div id="empty-folder" hidden>
                   <div class="image"></div>
                   <span class="label" i18n-content="EMPTY_FOLDER"></span>
                 </div>