How can an expression check for null values being passed through a rule input?

Using the isNull() function is the correct approach for checking whether null values are being passed through a rule input. This function directly evaluates a value and returns a boolean indicating whether the value is null. It provides a clear and straightforward means of validation, allowing developers to take appropriate actions based on the presence or absence of a value.

For example, if an expression receives user input from a form, utilizing the isNull() function can effectively determine if the user left a field empty or provided a null value. This is essential in scenarios where certain actions depend on the existence of valid input.

On the other hand, evaluating whether a value is greater than zero is not a reliable method for detecting null values since a null cannot be compared in this way; this will lead to errors or unexpected behavior. Using a local variable can encapsulate values for manipulation, but it does not directly assess for nullity. The a!refreshVariable function is meant for refreshing the value of a variable in the context of user interface components, but it does not serve the purpose of checking for null values.