Previously zero-length nop nodes used the lookahead start attributes
and current end attributes. This choice ends up being somewhat weird,
because the end attributes will be the at the last non-whitespace,
non-comment token, which might be quite far back. More problematically,
we may not have encountered any non-discarded token if we're at the
start of the file, in which case we will have no end attributes to
assign.
Change things to use a canonical "zero-length" node representation,
where the end position (token & file) will be exactly one before the
start position.
Fixes#589.
