Yeah, I don't really like that since the directory table indices are
meaningful in reading the raw DWARF. It doesn't really matter to how
we handle attr_value.source_file () or line table entries, since those
resolve it internally. But it doesn't feel quite right.
Anyway, as I said, we can figure it out more exactly later. For the
moment, let's just go with your hack to treat a NULL string as "" when