Start handling invalid stem directions more robustly

Introduce get_strict_grob_direction. This is like get_grob_direction, except that if it finds something that is not a direction, such as the empty list, it prints a warning and uses UP.

Convert a few calls to get_grob_direction to get_strict_grob_direction. I've done this when I felt comfortable that the change was correct. This is enough to fix the example in #6093 (closed), but the regtest suite is far from running completely with \override Stem.direction = #'(). Not sure wheter we should close #6093 (closed). On the long term, we should certainly use get_strict_grob_direction more often, but I am not confident enough to make invasive changes here.