Disable searching for no flagged messages

app/src/main/java/eu/faircode/email/AdapterFolder.java

@@ -349,6 +349,8 @@ public class AdapterFolder extends RecyclerView.Adapter<AdapterFolder.ViewHolder
                 tvFlagged.setText(NF.format(folder.flagged));
                 ibFlagged.setImageResource(folder.flagged == 0
                         ? R.drawable.baseline_star_border_24 : R.drawable.baseline_star_24);
+                tvFlagged.setEnabled(folder.flagged > 0);
+                ibFlagged.setEnabled(folder.flagged > 0);
 
                 tvError.setText(folder.error);
                 tvError.setVisibility(folder.error != null ? View.VISIBLE : View.GONE);