Fix crash in wxFileConfig when deleting last entry of the root group.

Fix crash in wxFileConfig when deleting last entry of the root group.

This resulted in keeping a dangling pointer to the group line in
wxFileConfigGroup and any attempt to use it after this resulted in a crash.
Fix this by explicitly resetting the last line in this case.

Also add a unit test for this scenario.

Closes #14243.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71311 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775