The `exists()` function refines a `ItemsQueryBuilder` to only match items where the value of the specified `propertyName` doesn't equal null or undefined.
 `exists()` checks for either existence or non-existence based on the boolean parameter. Note that `exists()` does match items where the value of the specified `propertyName` is an empty string or an invalid value. `exists()` is only useful for properties which don't contain default values and therefore their values may be unassigned.