Do not convert null values to zero in Decimal fields

Javascript Number converts null to 0, but the original value must be preserved
so we avoid converting null values to Decimal.

issue5846
review28711002

src/model.js

@@ -1692,7 +1692,10 @@
         },
         apply_factor: function(value, factor) {
             value = Sao.field.Numeric._super.apply_factor(value, factor);
-            return new Sao.Decimal(value);
+            if (value !== null) {
+                value = new Sao.Decimal(value);
+            }
+            return value;
         }
     });