LibGUI: Don't crash when rendering a collapsible toolbar with no items

This prevents a crash in GML Playground that would occur when
previewing a GUI with a collapsible toolbar.

Userland/Libraries/LibGUI/Toolbar.cpp

@@ -165,7 +165,7 @@ ErrorOr<void> Toolbar::create_overflow_objects()
 
 ErrorOr<void> Toolbar::update_overflow_menu()
 {
- if (!m_collapsible)
+ if (!m_collapsible || m_items.is_empty())
  return {};
 
  Optional<size_t> marginal_index {};