Remove extra separator menu item (doesn't show on Mac)

Fixes #3666

Auditors: @charbelrami @srirambv

Test Plan:
- With new code in place, launch Brave on Windows
- Delete all your bookmarks
- Confirm bookmarks menu looks good (no extra separator)
- Go to google.com
- Right click the "Google Search" button
- Confirm you don't see the extra separator

app/browser/menu.js

@@ -364,7 +364,7 @@ const createHistorySubmenu = (CommonMenu) => {
 }
 
 const createBookmarksSubmenu = (CommonMenu) => {
-  return [
+  let submenu = [
     {
       label: locale.translation('bookmarkPage'),
       type: 'checkbox',
@@ -386,9 +386,16 @@ const createBookmarksSubmenu = (CommonMenu) => {
     CommonMenu.bookmarksManagerMenuItem(),
     CommonMenu.bookmarksToolbarMenuItem(),
     CommonMenu.separatorMenuItem,
-    CommonMenu.importBookmarksMenuItem(),
-    CommonMenu.separatorMenuItem
-  ].concat(menuUtil.createBookmarkMenuItems())
+    CommonMenu.importBookmarksMenuItem()
+  ]
+
+  const bookmarks = menuUtil.createBookmarkMenuItems()
+  if (bookmarks.length > 0) {
+    submenu.push(CommonMenu.separatorMenuItem)
+    submenu = submenu.concat(bookmarks)
+  }
+
+  return submenu
 }
 
 const createWindowSubmenu = (CommonMenu) => {

js/contextMenus.js

@@ -870,7 +870,11 @@ function mainTemplateInit (nodeProps, frame) {
       label: locale.translation('redo'),
       accelerator: 'Shift+CmdOrCtrl+Z',
       role: 'redo'
-    }, CommonMenu.separatorMenuItem, ...editableItems, CommonMenu.separatorMenuItem)
+    }, CommonMenu.separatorMenuItem)
+
+    if (editableItems.length > 0) {
+      template.push(...editableItems, CommonMenu.separatorMenuItem)
+    }
   } else if (isTextSelected) {
     if (isDarwin) {
       template.push(showDefinitionMenuItem(nodeProps.selectionText),