In that case you shouldn't use the validation script shown above and do everything in the calculation. For example, if the result of the calculation can have 0 as a valid result, and you want to only show a result if both of the fields have a value, the custom calculation script could be:
// Custom calculation script
(function () {
// Get the field values, as strings
var s1 = getField("Text1").valueAsString;
var s2 = getField("Text2").valueAsString;
// Perform the calculation only if both fields have are non-blank
if (s1 && s2) {
event.value = +s1 - +s2;
} else {
event.value = "";
}
})();
or more compactly:
(function () {
var s1 = getField("Text1").valueAsString;
var s2 = getField("Text2").valueAsString;
event.value = s1 && s2 ? +s1 - +s2 : "";
})();