deposit: doi syntax validator improvements

NOTE the name of the validator was changed to DOISyntax validator
because of convertion to a class. 

@lnielsen-cern I think you use it, so you need to adapt the code.
> No, in case doi is empty it should not raise an exception.

do you think that `doi` can be an empty string? Then: `if doi and not self.regexp.match(doi):`