message: Fix error that causes 0 values to be the empty string
  the `value || ""` replacement was making 0 values replaced by '' (the empty string) instead of keeping 0 which is a valid value.
diff: @@ -2780,7 +2780,7 @@ var PptxGenJS = function(){
  strXml += ' <c:numCache>';
  strXml += ' <c:formatCode>General</c:formatCode>';
  strXml += ' <c:ptCount val="'+ obj.labels.length +'"/>';
  - obj.values.forEach(function(value,idx){ strXml += '<c:pt idx="'+ idx +'"><c:v>'+ (value || '') +'</c:v></c:pt>'; });
  + obj.values.forEach(function(value,idx){ strXml += '<c:pt idx="'+ idx +'"><c:v>'+ value +'</c:v></c:pt>'; });
  strXml += ' </c:numCache>';
  strXml += ' </c:numRef>';
  strXml += ' </c:val>';
  