Returns the cache key segment that identifies the user an entry was loaded for, or null when
there is no user. A null result means the data must not be cached at all, because any key built
without a user would be shared by every caller.
Parameters
userID: string|null|undefined
the ID of the user the data was loaded for
Returns string|null
u:<normalized user ID>, or null when userID is empty
Returns the cache key segment that identifies the user an entry was loaded for, or
nullwhen there is no user. Anullresult means the data must not be cached at all, because any key built without a user would be shared by every caller.