Change empty value for snapshot_id from '' to nil.
Change empty value for snapshot_id from '' to nil.

Before this change, using the ebs_raid resource without a `snapshots`
attribute produced the following error:
> Aws::EC2::Errors::MissingParameter
> The request must contain the parameter size/snapshot

Fixes #118.

BuyerQuest/aws,needle-cookbooks/chef-opscode-aws,grosendorf/aws,chef-cookbooks/aws,paybyphone/aws,rafaelfelix/aws,Tealium/aws,strebitz/aws,udomsak/aws,joshoohaah/aws,evertrue/aws,incent-corp/aws,arnaud036/aws,itwasntandy/aws,localytics/aws,opscode-cookbooks/aws,miketheman/aws-cookbook,Altiscale/aws,xacaxulu/aws,TheWeatherCompany/aws,ForeverOceans/aws,scalp42/aws