Your problem is not ConfigurationManager.GetSection(sectionName) returns null, it is how can I test some code containing ConfigurationManager.GetSection(sectionName)?
And the answer is: wrap it, inject it, then for your test mock it.
You have several examples of pepole facing the same issue:
- http://chrisondotnet.com/2011/05/configurationmanager-wrapper-for-unit-testing/
- http://weblogs.asp.net/rashid/archive/2009/03/03/unit-testable-configuration-manager.aspx
(The second one is much more detailed, still the idea is the same).
Anyway, this is quite logical that you cannot use information from app.config in a unit test, as an app.config is contextual for the whole application, when it is required to write test absolutely independant. If you use directly an app.config value, then you have non logical coupling.