Let ActiveStorage tests pass only for Disk

[claudiob]

If you have a "service/configurations.yml" file, but every single line is
commented out, then an error occurs when running tests:

git:active-storage-import~/code/rails/activestorage$ rake
~/code/rails/activestorage/test/test_helper.rb:17:in `<top (required)>': undefined method `deep_symbolize_keys' for false:FalseClass (NoMethodError)
	from ~/code/rails/activestorage/test/controllers/direct_uploads_controller_test.rb:1:in `require'

The reason is that YAML.load(..an empty file content..) simply returns false, and not {}.

This PR fixes this behavior so tests can also run when no remote service
is available.

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[rails-bot]

Warning

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