Determines if the input flow is NULL or its length = 0.
A NULL flow is flow with a length of -1. it can be generated by a flow-pass, flow-stop, flow-pass-if-changed.
A ZERO length flow can also be generated by a array-set-size,
Returns a 1 value if the input flow has a ZERO length.
Returns a 1 value if the input flow has a NULL length.
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