Issues caused by attributes with undefined values

[Reinmar]

#500 showed that it may happen that a slightly broken converter will try to create an element in the model which has some attribute but the value of this attribute is undefined.

There's a range of issues which such attributes may cause. It can't be stringified (JSON.stringify() will skip it) so it may blow up collaborative editing servers. It causes issues on conversion to the view because (that's my hypothesis) proper attribute elements are created but they are not merged later on for some reasons (the second part of #500).

I wonder if we shouldn't reject such attributes at some point because such issues may be hard to track. Perhaps, once will have "one API to rule them all", the right place will be in the model writer? Or perhaps this should be done directly in Node?

[Reinmar]

Another thing is that we could actually define what kind of falsy values the model handles. I think that null, 0, '' and false work fine. It'd be good to have some policy about that.

[Reinmar]

This problem isn't hitting back, so let's assume it does not exist.