Fix: Open new tab when last unpinned tab is closed in workspace and no pinned tabs are loaded

Added a preference to keep default behavior or apply the new one.

src/browser/components/preferences/zen-settings.js

@@ -1066,4 +1066,9 @@ Preferences.addAll([
     type: 'bool',
     default: true,
   },
+  {
+    id: 'zen.workspaces.open-new-tab-if-last-unpinned-tab-is-closed',
+    type: 'bool',
+    default: true,
+  },
 ]);