Treat whitespace strings as empty() in ValueChecks.

* Treats strings that are only whitespace as "empty" in the funtion `ValueChecks.empty()`.

Note that this change could be quite controversial. However, consider the naming of `empty` and what we actually mean by it. Let's say this is used to validate whether a user has entered anything into a textbox or not. The user would probably agree that she did not enter anything if it only contains a single space. Another example would be to validate whether some http response from a service actually contained anything. If there's only whitespace returned from the service, it's more likely we would think of that response as "empty" than "contains an actual response".
Merge pull request #61 from iZettle/treat-whitespace-strings-as-empty

Treat whitespace strings as empty() in ValueChecks.