[aws|compute] Fixes schema in image tests

The schema for the AWS ImagesSet declared blockMappingDevice as an Array
with no values inside. Mocks are generating an image with a device
present which no longer fits the schema.

This is a premptive fix because corecting the #formats method caused this
issue to surface. See #1477

tests/aws/requests/compute/image_tests.rb

@@ -2,7 +2,7 @@ Shindo.tests('Fog::Compute[:aws] | image requests', ['aws']) do
   @describe_images_format = {
     'imagesSet'    => [{
       'architecture'        => String,
-      'blockDeviceMapping'  => [],
+      'blockDeviceMapping'  => [Fog::Nullable::Hash],
       'description'         => Fog::Nullable::String,
       'hypervisor'          => String,
       'imageId'             => String,
@@ -135,4 +135,4 @@ Shindo.tests('Fog::Compute[:aws] | image requests', ['aws']) do
       Fog::Compute[:aws].modify_image_attribute('ami-00000000', { 'Add.UserId' => ['123456789012'] }).body
     end
   end
-end
+end