Fix bug in string output of empty array

[Thirdegree]

>>> from tomlkit import item, dumps, loads
>>> t = item([])
>>> t.multiline(True)
[]
>>> print(dumps(t))
[
    ,
]
>>> loads(dumps(t))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/thirdegree/.virtualenvs/tmp-b9dbc870a56cb56/lib/python3.6/site-packages/tomlkit/api.py", line 34, in loads
    return parse(string)
  File "/home/thirdegree/.virtualenvs/tmp-b9dbc870a56cb56/lib/python3.6/site-packages/tomlkit/api.py", line 51, in parse
    return Parser(string).parse()
  File "/home/thirdegree/.virtualenvs/tmp-b9dbc870a56cb56/lib/python3.6/site-packages/tomlkit/parser.py", line 160, in parse
    key, value = self._parse_table()
  File "/home/thirdegree/.virtualenvs/tmp-b9dbc870a56cb56/lib/python3.6/site-packages/tomlkit/parser.py", line 1076, in _parse_table
    name_parts = tuple(self._split_table_name(name))
  File "/home/thirdegree/.virtualenvs/tmp-b9dbc870a56cb56/lib/python3.6/site-packages/tomlkit/parser.py", line 267, in _split_table_name
    raise self.parse_error()
tomlkit.exceptions.ParseError: TOML parse error at line 3 col 0
>>>