When a form has a numeric field, it initially is blank (assuming you have not given it a default value). However at runtime, once you give it a value, you are not able to clear the field again.with a script unless you click the reset button or reload the form. Unfortunately, that will clear all the fields in the form, not just the one field.
You can compare the value of a numeric field to null, you cannot assign a null to a numeric field. The best you can do is give the field a zero value.
However, I found that you can clear a numeric field by doing the following:
input.NumericFieldName = null.toDecimal();
or
input.NumericFieldName = null.toLong();
Maybe you could make a calculated field with a case statement that replaces the nulls with 0? That would probably fix the confusion on the dashboard side.