Fix CompoundCurve::validateConstruction for curves with empty elements

[dbaston]

          I may miss something, but it is not obvious to me that you can't insert an empty curve in a CompoundCurve, in which case back() and front() are returning something invalid. At least if using directly the CompoundCurve::CompoundCurve(std::vector<std::unique_ptr<SimpleCurve>>&& p_curves,                              const GeometryFactory& gf) constructor. That might be hard/impossible to trigger through WKT.  But maybe through WKB ?

Originally posted by @rouault in #1164 (comment)