`GetPluginFiles` method must not throw InvalidOperationException

[DevD4v3]

Implementations:

• CPluginEnvConfiguration.GetPluginFiles
• CPluginJsonConfiguration.GetPluginFiles

This exception occurs when the Plugins key does not exist in a configuration file as a json or .env.
This behavior should be changed for two reasons:

• Violates the Principle of least astonishment. If the consumer does not want to load any plugins, then it should delete the configuration key, i.e. the Plugins key found in a configuration file and that's it.
  The expected behavior for the consumer is that no plugin is loaded, but the opposite happens: an exception is thrown.
  This surprises the consumer.
• As the consumer has been surprised, then should create the configuration key with an empty value. However, if the consumer uses a .json file as source, an exception will still be thrown if an empty array appears in the Plugins section.
  See issue Cannot specify an empty array in the Plugins section of appsettings.json #6

If the consumer forgets to add the Plugins key in the configuration file, then the expected behavior is that no plugins are loaded. This is not a bug.