Substitution of zero for null database entries.

When my data values are exported to PVOutput for gas consumption there are null values included after flow stops. It appears that for around 30 minutes after flow stops and no more pulses are forthcoming the records just have nothing recorded until after the 30 minute period the value of zero is recorded continuously until flow starts again. This causes incomplete graphs in PVOutput.
Is there some way to force these null valued to be substituted for a zero?