bpo-29412: patched string index out of range error in get_word function of _head…

[TyrannosourceExe]

…er_value_parser.py and created tests in test__header_value_parser.py for empty string.

bpo-29412: patched string index out of range error in get_word function of _header_value_parser.py by checking for empty string.  Empty string is returned by get_cfws(value)[1] when everything is CFWS.

Backport Pull Request title

Changes suggested to be applied to python 3.5 , 3.6 , and 3.7

https://bugs.python.org/issue29412

[the-knights-who-say-ni]

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[TyrannosourceExe]

I verified that I signed the CLA. let me know if there is anything else I need to do! :)

[zooba]

Should be no need to copy these two lines from below.

[bebarino]

I'm running into this bug in the alot package (the pazz/alot#1367 referenced bug above). I'm using python 3.5 so I've backported this fix there and I run into another issue where the backtrace ends like this:

    values = mail.get_all(key)
  File "/usr/lib/python3.5/email/message.py", line 511, in get_all
    values.append(self.policy.header_fetch_parse(k, v))
  File "/usr/lib/python3.5/email/policy.py", line 160, in header_fetch_parse
    return self.header_factory(name, value)
  File "/usr/lib/python3.5/email/headerregistry.py", line 586, in __call__
    return self[name](name, value)
  File "/usr/lib/python3.5/email/headerregistry.py", line 197, in __new__
    cls.parse(value, kwds)
  File "/usr/lib/python3.5/email/headerregistry.py", line 344, in parse
    for mb in addr.all_mailboxes]))
  File "/usr/lib/python3.5/email/headerregistry.py", line 344, in <listcomp>
    for mb in addr.all_mailboxes]))
AttributeError: 'Group' object has no attribute 'local_part'

So this change isn't fixing everything for me, but maybe that's because on python 3.5 there's something else missing? The header that's failing is

To: unlisted-recipients:; (no To-header on input)

in case that helps here.

[meskio]

I have tested the patch (removing the two lines mentioned by @zooba) and it fixes the issue for me (using python 3.7).

@TyrannosourceExe Do you mind removing this two lines? I will love to see this merged.

[maxking]

Since @TyrannosourceExe hasn't responded in a while, I am willing to pick up this patch and get it merged. I'll wait for a week or though if @TyrannosourceExe wants to work on the comments.

[meskio]

@maxking thanks for picking this up. I was hitting it daily with my email client, I end up patching the email library in my system, and after 4 months using this patch I haven't have problems.

[csabella]

Merged under GH-14387. Thanks, @maxking!