fix(headless): Do not create an empty session in app context on logout

[joonhyungshin]

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Description

This commit complements #3785 . Even though the headless API won't attach a session token when user logs out, the empty session is still created in the database upon exiting the context manager authentication_context(). This patch fixes this by also checking if the session is empty within the context manager.

Note: this behavior is quite tricky to test, because normally there is no way to access the 'ghost' session. If the session backend is guaranteed to be database-backed, then one way is to check the number of sessions before and after logout. The test code may be slightly messy though - I'll add a test if requested.

[coveralls]

coverage: 95.545%. remained the same
when pulling cc4d560 on joonhyungshin:fix-empty-session
into 2a9790a on pennersr:main.