FileBasedWebSessionCacheTest: Pass test upon invalid key

Before this change, getIfPresentInvalidKeyTest passed upon
getIfPresent returning null out of key not being a String. Pass the
String equivalent of the latter instead, so that getIfPresent returns
the null result of a failing web session resolution, as test expects.

Fix the Eclipse warning caused by the previous casting usage.

Change-Id: I068793f86839a45e9a963be9b6f353afdb971386
1 file changed