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).
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