Don't validate type for blank values

Another case where a validation is implying ValuePresence when it shouldn't be.
This should be the last case as now all the nodes that use ValuePresence have
been fixed.
`blank?` isn't core Ruby; it's provided by `ActiveSupport`: http://api.rubyonrails.org/classes/Object.html#method-i-blank-3F
I actually note you're using it elsewhere in the gem as well, and I'm not sure how the tests are passing without it being defined. Presumably for the type checker you don't want to allow Nil values, so `nil?` (which is defined on `Object`) is no good for you?