[TASK] Early return and NOTICE in ConfigurationService on bad usage

This change sends a debug message and returns and empty array early if the ConfigurationService->getPageConfiguration method is called with an invalid (empty but not NULL) value as $extensionName argument.

Fixes: #71
Merge pull request #74 from NamelessCoder/emptyextension

[TASK] Early return and NOTICE in ConfigurationService on bad usage