As part of fixing bpo-27931 code was introduced to get_bar_quoted_string
that added an empty Terminal if the quoted string was empty.  This isn't
the best answer in terms of the parse tree, we really want the token
list to be empty in that case.  But having it be empty would result in
local_part raising the index error.  Which is the same bug we find if we
try to parse an address consisting of a single dquote.  So by fixing
local_part to not raise in that case, we can have the bare_quoted_string
code correctly return an empty token list for the empty string cases
(two dquotes or a single dquote as the entire addrespec, at the end of a
line).

After this commit there will be two test failures instead of just one,
and we'll fix both with the fix to local_part.