[lldb/DWARF] Simplify DIE extraction code slightly

Remove the "depth" variable, as the same information can be obtained
through die_index_stack.size().

Also add a test case for a one tricky case I noticed -- a unit
containing only a null unit die.

GitOrigin-RevId: 1b96e133cf5215cb9ebfe7f14630f479c1611f22
1 file changed