AllowedValue constraints incorrectly handles null value

[Peter Razumovsky]

Let our template will be next:

heat_template_version: newton

resources:
  rs:
    type: OS::Heat::RandomString
    properties:
      sequence: null

sequence property has no default value, so it will be equals to "".

If I try to create stack, I get next error:

ERROR: Property error: : resources.rs.properties.sequence: : "" is not an allowed value [lettersdigits, letters, lowercase, uppercase, digits, hexdigits, octdigits].

That's incorrect at the point of user, because I specify null property, there's no "" value - such error confuses. Need to fix such behaviour.