G4Polyhedra interprets negative/zero angle dphi as twopi.

This fixes this test of Geant4 built using VecGeom. The test fails due to streaming messages like Quadrilaterals::Set: could not find three non-degenerated points to stderr, but the error is in fact due to invoking the G4Polyhedra constructor with totalPhi = 0, interpreted by Geant4 as TwoPi but as 0 by vecgeom::UnplacedPolyhedron. This creates degenerated inner and outer quadrilaterals, and is not intercepted by the assert(dPhi > 0) hidden behind NDEBUG.