Use empty+nil instead of blank, fix issue with gp2 provisioning

- Replaces `blank?` with `empty? || nil?`
- Don't cast `nil` to `0` for `iops`, since the AWS SDK rejects `gp2` provisioning where an `iops` parameter is present.
Merge pull request #57 from sandstrom/iops

Use empty+nil instead of blank, fix issue with gp2 provisioning