You can use the test for null to ignore whatever parameter was not set, i.e. no filter on.
AND (@BU is null or ADP_FEED_0318.BU= @BU) --- BU. if it is null, it passes the check, otherwise must match filter
AND (@Approver is null or ADP_Feed_0318_2.FirstName + ' ' + ADP_Feed_0318_2.LastName = @Approver)
.. etc