getElementValue function
Gets the element value depending of identified type.
If the resolved value is null or empty, and def is not null,
it will return def.
Implementation
String? getElementValue(Element element, [String? def]) {
String? value;
if (element.isA<HTMLInputElement>()) {
value = (element as HTMLInputElement).value;
} else {
value = getElementSRC(element);
}
return def != null && isEmptyObject(value) ? def : value;
}