Hi Ferran -
Instead of the raster calculator, I would use the CON geoprocessing tool in the Spatial Analyst toolbox. The syntax for the GP tool would be as follows:
in_conditional_raster = your input raster
in_true_raster_or_constant = 0
in_false_raster_or_constant = your input raster
where_clause = VALUE IS NULL
If the raster cell evaluates as TRUE (i.e. IS NULL), the cell gets a value of 0. Else if the raster cells evaluates FALSE (i.e. IS NOT NULL), then the raster cell gets the original input raster cell value.