don't set python exe to undefined

Summary:
FindPythonInterp sets PYTHON_EXECUTABLE, we overrode it by setting an
empty var. add a comment to explain the unbalancedness of the conditional
branches.

Test Plan: pythoncairo is correctly found with cmake 3.10.2

Reviewers: davidedmundson, fvogt

Reviewed By: fvogt

Subscribers: fvogt, heikobecker, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18309