DWARF: Never emit (0, 0) to mean an empty span in debug_loc (#2940)

After mapping to the new positions, and after relativizing to the
base, if we end up with (0, 0) then we must emit something else, as
that would be interpreted as the end of a list. As it is an empty
span, the actual value doesn't matter, it just has to be != 0.
This can happen if the very first span in a compile unit is an
empty span, in which case relative to the base of the compile unit
we would have (0, 0).

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