gh-148518 fix index error in local part attribute (#148522)

As part of fixing bpo-27931 code was introduced to get_bare_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 resulted in
local_part raising the index error.  We find that same problem if we
try to parse an address consisting of a single dquote.  By fixing
local_part to not raise on an empty token list, 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).