selectFieldOption breaks if null value passed

[DonCallisto]

I think that we should normalize the value to empty string if null is passed.
If you need a working example, I'll provide it in the week end but it is pretty clear looking at
https://github.com/minkphp/Mink/blob/master/src/Selector/NamedSelector.php#L208
and
https://github.com/minkphp/Mink/blob/master/src/Selector/NamedSelector.php#L224

Basically, %locator% are never replaced and so no possible way to produce a valid xpath.

If value normalization is note feasabe (as empty string could be a valid use case?) we should throw an exception.

If you all agree to replace null with empty string (or to throw an exception), I'll be happy to make a PR.