fixes for potential crash / invalid operations on radial fracture of pillar
- make 'embed surface path' find the intended start vertex for its path when it's known
- guard against path being empty before accessing element 0
- guard against invalid edge collapse in mesh boolean that was triggered by this example
#rb david.hill
#rnx
#ushell-cherrypick of 16760529 by Jimmy.Andrews
#preflight 60d49b4557b30200011a0594
[CL 16775195 by Jimmy Andrews in ue5-main branch]