Fix issue importing TRIANGULATED_FACE from STEP where there are no pnval entries

When testing, we encountered an issue importing some TRIANGULATED_FACE entries from a step file.

Currently, OCCT is unable to import entries where no pnindex identifiers are specified. In this case, it seems like the correct behaviour is to fallback to indexes in the underlying coordinate set.

The entries that fail to import look as below;

#0000466=COORDINATES_LIST('',4,((-157.814575,-532.5,616.),(-157.814575,-167.5,616.),(-25.814579,-532.5,616.),(-25.814579,-167.5,616.)));
#0000467=TRIANGULATED_FACE('',#0000466,4,((0.,0.,1.)),$,(),((1,3,4),(4,2,1)));
#0000468=TESSELLATED_SHELL('',(#0000467),$);

Here is the relevant entry from the standard
https://www.steptools.com/stds/smrl/data/modules/tessellated_geometry/sys/4_info_reqs.htm#tessellated_geometry_arm.triangulated_face

Attached is a patch that fixed the behaviour in internal testing, and an example step file that fails to import with OCCT 7.7.1