Fix undefined variable and write unit tests for OC_Principal connector

https://github.com/owncloud/core/commit/76b705602629e8ef6ff19176bcd81d92c9cdcad6 fixes the error in #12681

@schiesbn @derkostka Please test and review.
i.e. now this value is static and cannot be changed by the user, I have no idea what clients / APIs rely on the fact that this always returns the same value.