🐛 Unable to Destroy Create Failed Bucket

[computeracer]

• Fixes bug ERROR]: no implicit conversion of nil into String #2
  where the callback was unable to destroy a stack that contains
  a bucket in the 'CREATE_FAILED' state.

To Reproduce:

1. Create a stack with an improper bucket name.

SparkleFormation.new(:bucket_test, :provider => :aws) do

  dynamic!(:bucket, :bucket_one) do
    depends_on 'Bucket2Bucket'
    properties do
      bucket_name 'Bad-bucketname2334'
    end
  end

  dynamic!(:bucket, :bucket2) do
    properties do
      bucket_name 'test122354'
    end
  end
end

2. Try to destroy the stack with sfn_bucketpurge 0.0.1 enabled, and find
   the following error along with remaining stack.

[ERROR]: no implicit conversion of nil into String

[computeracer]

@chrisroberts would you have time to review this small bug fix?