stringify: return `undefined` if impossible to stringify BEM notation

[blond]

now need use try..catch:

try {
    var str = bemNaming.stringify({ elem: 'elem' });
} catch(e) { /* ... */ }

it will be easier to check for an empty string:

var str = bemNaming.stringify({ elem: 'elem' });

if (str) {
    /* ... */ 
}

[tadatuta]

I think semantically it should be undefined (like in sentence «BEM entity is undefined»)

[blond]

I think semantically it should be undefined (like in sentence «BEM entity is undefined»)

yep, it's better