If you need the data for a field to show as a zero instead of a null value, you can use calculated columns to convert it.
The basic formula is:
CASE WHEN <yourfieldname> IS NULL THEN 0 ELSE <yourfieldname> END
Here's an example using the 'Comments' count field from Facebook Insights:
This technique is also necessary when using metrics to create custom calculations for your reports - if any value in the math is null, it will turn the whole result null! To get around this, you can create a new calculated field for each metric with the above formula, and then use these fields instead to build your custom calculations. With the nulls as zeros, it will do the math correctly.