Bugfix: ActiveSupport::EncryptedConfiguration reading of comment-only encrypted files (#34014)

* Fix reading comment only encrypted files

When a encrypted file contains only comments then reading that files raises an error:

    NoMethodError: undefined method `deep_symbolize_keys' for false:FalseClass
        activesupport/lib/active_support/encrypted_configuration.rb:33:in `config'
        test/encrypted_configuration_test.rb:52:in `block in <class:EncryptedConfigurationTest>'

This happens because the previous implementation returned a `{}` fallback for blank YAML strings. But it did not handle YAML strings that are present but still do not contain any _usefull_ YAML - like the file created by `Rails::Generators::EncryptedFileGenerator` which looks like this:

    # aws:
    #   access_key_id: 123
    #   secret_access_key: 345

* Fix coding style violation

* Add backwardscompatible with Psych versions that were shipped with Ruby <2.5

* Do not rely on railties for Active Support test

* Simplify error handling

* Improve test naming

* Simplify file creation in test

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