When using the Raster Calculator in ArcToolbox (version 10.3.1) the Con function is expressed as follows for a raster named "streams":

Con(IsNull("streams"), 0, "streams")

this returns zero if streams is null else it returns the value of streams.

In this example all NoData pixelsin the Streams are set to zero, otherwise they contain the pixel value found in the Streams dataset.

When using the Raster Calculator in ArcToolbox (version 10.3.1) the Con function is expressed as follows for a raster that is named "streams":

Con(IsNull("streams"), 0, "streams")

this returns zero if streams is null else it returns the value of streams.

In this example all NoData pixels in the Streams are set to zero, otherwise they contain the pixel value found in the Streams dataset.