I'm posting a solution to an issue I had with inserting a zero when an integer or decimal question type was blank in a report template. Hope this saves someone hours of troubleshooting, despair, and frustration. 
The following expression worked for me on a feature report for a survey that had nested repeats (e.g. Project main-->Site repeat-->Feature repeat). In my case, the report ran calculations on all the features within a single project. If the question is blank a zero is inserted; otherwise, a calculated value is returned.
Syntax:
${#FeatureRepeat | stats:"sum,Question name to sum,Variable name to hold sum result"}${Variable name+0 | format:"#,##0"}${/}
Example:
${#rptFeature | stats:"sum,RoadMilesDecomm,RdMi"}${RdMi+0 | format:"#,##0"}${/}
The variable name can be whatever you want it to be. The number format is optional.