Fill cells with null if amount is zero

RhSolutions.AddIn/Services/ExcelWriterBase.cs

@@ -134,7 +134,14 @@ namespace RhSolutions.Services
                 }
                 else
                 {
-                    cell.Value2 = amount;
+                    if (amount == 0)
+                    {
+                        cell.Value2 = null;
+                    }
+                    else
+                    {
+                        cell.Value2 = amount;
+                    }
                 }
             }
         }