would you like to show only the records with Field A = Pippo and Field B = '' OR you want to have your table with all the rows and just when you push a button reduce the number of rows in according to your rules?
You can't select nulls so a Select in Field action for field B won't work. But perhaps you can use a calculated dimension to achieve what you want. Mabye something like: if([field A] = 'PIPPO' and len(trim([field B]))=0, [field A]). And enable the option Suppress When Value Is Null for the calculated dimension.
Of course, a good practice is to replace nulls during the load with some value that can be selected. You can use the NullAsValue statement and NullValue variable. The two lines below would replace all nulls with the string 'Null' if you place them at the start of the script.