Example using DataFu's COALESCE to replace a value with zero if it is null.

coalesce_null.pig

define COALESCE datafu.pig.util.Coalesce();
  
data = LOAD 'input' using PigStorage(',') AS (val:INT);
  
dump data;
-- (1)
-- ()
  
data2 = FOREACH data GENERATE COALESCE(val,0) as result;
  
dump data2;
-- (1)
-- (0)