Invalid dumps output when appending to a multiline array

[abravalheri]

Hello, I noticed an unexpected output (and invalid TOML syntax) for dumps, when trying to append a dict object to an existing multiline array. Maybe this should be considered a bug?

Please find bellow the steps to reproduce the error:

>>> import tomlkit
>>> example = """\
... x = [
... ]
... """
>>> doc = tomlkit.loads(example)
>>> doc["x"].append({"name": "John Doe", "email": "john@doe.com"})
>>> doc
{'x': [{'name': 'John Doe', 'email': 'john@doe.com'}]}
>>> print(tomlkit.dumps(doc))
x = [
, name = "John Doe"
email = "john@doe.com"
]

Here we can see that the value dumped for x does not conform with the TOML syntax. I would expect the output of dumps to be similar to the following:

x = [
   {name = "John Doe", email = "john@doe.com"}
]

Please notice that even when using explicitly inline_table, there still will be errors:

>>> doc = tomlkit.loads(example)
>>> t = tomlkit.inline_table()
>>> t.append("name", "John Doe")
{'name': 'John Doe'}
>>> t.append("email", "john@doe.com")
{'name': 'John Doe', 'email': 'john@doe.com'}
>>> doc["x"].append(t)
>>> doc
{'x': [{'name': 'John Doe', 'email': 'john@doe.com'}]}
>>> print(tomlkit.dumps(doc))
x = [
, {name = "John Doe", email = "john@doe.com"}]

Here the extra initial , will cause problems when parsing the produced string.

I am using Python 3.8.0 and tomlkit 0.7.2 on Ubuntu 18.04.5 LTS.