Use TargetNullValue in binding expressions.

You can replace null values in your data with a target value using TargetNullValue in the Binding Property.

For e.g. the Text property of the TextBlock can be formatted to a format specified below:
   <TextBlock Text=""{Binding Sum, TargetNullValue=0.0}"" />

Note: This is available as part of 3.5 SP1.