Revert "Map user 0 to current user"

This reverts commit 1faed58ec1050a855240a0bf4be018a3f2cda019.

Previous commit was introduced due to some tests failing, but actually
the tests were wrongly coded.
Having a filter to be executed as NO_USER currently is not supported. I
prefer let it raise an error when some of the filter operations makes no
sense, than defaulting to current user thus potentially hiding higher
level conceptual errors.