When you create a pivot table and if a source cell has no value (i.e. cell is blank), in pivot table, it will show a blank. All other results in pivot table are carrying numeric values and you see blanks. It is odd though it doesn't impact the results.
But you still prefer to show this as 0.
1. Click anywhere in the pivot table and choose PivotTable Options.
2. Make "For empty cells show" box 0.