Don't try to extract accelerators from menu items which don't have any.

Don't try to extract accelerators from menu items which don't have any.

We don't need to call wxAcceleratorEntry::Create() in wxMenuItem::GetAccel()
if the menu item doesn't have any accelerator at all, i.e. if there is no TAB
in its label.

Calling wxAcceleratorEntry::Create() is useless and won't work correctly any
more when Create() is updated to allow passing it strings without TAB in them
in the next commit.

See #12745.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66307 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775