Skip language option tags without value at AtCoder

[serihiro]

What does this PR change?

This PR fixes a parse error when parsing available languages from AtCoder submit pages.

Background

When I tried to submit my code to AtCoder, I faced the following error today.

$ oj -v s https://atcoder.jp/contests/abc055/tasks/abc055_b main.cpp

...

Traceback (most recent call last):
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/_implementation/main.py", line 233, in main
    run_program(namespace, parser=parser)
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/_implementation/main.py", line 212, in run_program
    submit(args)
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/_implementation/command/submit.py", line 56, in submit
    language_dict = {language.id: language.name for language in problem.get_available_languages(session=sess)}  # type: Dict[LanguageId, str]
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/service/atcoder.py", line 869, in get_available_languages
    data = self.download_data(session=session)
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/service/atcoder.py", line 804, in download_data
    return AtCoderProblemDetailedData.from_html(html, problem=self, session=session, response=resp, timestamp=timestamp)
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/service/atcoder.py", line 760, in from_html
    available_languages = cls._parse_available_languages(soup, problem=problem)
  File "/Users/kazuhiroserizawa/.pyenv/versions/3.8.0/lib/python3.8/site-packages/onlinejudge/service/atcoder.py", line 728, in _parse_available_languages
    languages += [Language(option.attrs['value'], option.string)]
KeyError: 'value'

[ERROR] 'value'

This error was caused by the modification of the Form of ActCoder's submit pages.
The language select list has an empty option tag for search now.

Because current implementation of oj does not expect option tags without value element, key error occurs when parsing the empty option tag.

This PR modifies the implementation to skip empty option tags.

[serihiro]

Two test cases failed at CI 🤔
But I think these failures are not related to this PR.

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=========================== short test summary info ===========================
FAILED tests/service_codeforces.py::CodeforcesProblemTest::test_download_problem
FAILED tests/service_yukicoder.py::YukicoderOfficialAPITest::test_get_user_0
= 2 failed, 228 passed, 36 skipped, 2 xfailed, 10 warnings in 195.97s (0:03:15) =
Command exited with code 1

[kmyk]

LGTM
I could reproduce the error and confirmed that this pullreq fixes it.

Thank you for a nice pull request!

[kmyk]

CI fails, but not related for this problem. I'll fix it and merge this soon.