STATUS_NULL indicates that no output was produced, but there may be more
results. This can happen if a value is filtered out or an empty bag is
flattened. A caller will typically ignore the output and try again after
This does NOT indicate that the output is the value 'null' (which is
possible in expressions). This is represented as 'null' with STATUS_OK.
STATUS_EOP indicates that no output was produced, and no further outputs
will be produced (e.g. all attached inputs have been consumed or a limit
has reached its threshold). A caller will typically terminate or attach
new inputs on seeing this status.