Raise legible error when trying to emit TOML with null values

[kislyuk]

TOML does not support null values (not sure how that makes any sense, but apparently it never will: toml-lang/toml#30)

We currently raise the following error when encountering them: tomlkit.items._ConvertError: Invalid type <class 'NoneType'>

Come up with a more legible error that makes it more obvious what's going on.