Test is erroring with

dataLayerHelper
generateProductLinesDataLayer
extends the datalayer:
ReferenceError: empty is not defined
at Object.generateProductLinesDataLayer (cartridges\gtm\int_gtm_sfra_custom\cartridge\scripts\gtm\dataLayerHelper.js:42:9)
at Context.<anonymous> (cartridges\test\unit\gtm\int_gtm_sfra_custom\scripts\dataLayerHelper.js:59:42)
at processImmediate (internal/timers.js:464:21)
at process.callbackTrampoline (internal/async_hooks.js:131:17)

Here:

if (!empty(pli.product) && pli.product.isVariant()) {
    pid = pli.product.masterProduct.ID;
    variant = pli.productID;
}

Why would I get an error when testing a method that uses a built in empty() method that doesn't require any other reference?

Test:

describe("generateProductLinesDataLayer", function () {
        it.only(" extends the datalayer", function () {
            var result = dataLayerHelper.generateProductLinesDataLayer(productLineItems);
            assert.deepEqual(result, []);
        });
    });

function generateProductLinesDataLayer(productLineItems) {
......
        if (!empty(pli.product) && pli.product.isVariant()) {
            pid = pli.product.masterProduct.ID;
            variant = pli.productID;
        }
......
}