Add empty InsetLayout for undefined cases. Should avoid possible bugs caused by empty...

Add empty InsetLayout for undefined cases. Should avoid possible bugs caused by empty layout_ pointer in InsetCollapsable.

NOTE: Some cleanup is needed here, and I'll do it shortly. Doing it properly, though, requires making InsetLayout a proper class. (At the moment, it's just a C-style struct.) That should be committed separately.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@23103 a592a061-630c-0410-9148-cb99ea01b6c8