Component.create() misbehavior if Component.default_config is empty

[Philippe Normand]

Code issue.

In Component.create() we currently set the instance.config attribute properly only if the class has a non-empty default_config.

In the case where default_config is empty and config_section is None, the config attribute should be set to an empty elisa.core.config.Config section. When the config is written back to disk, all empty sections should be skipped, as a side-effect.