How to pass a empty string to a function argument

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Java8

I need to test a function in junittestcase my function is:

public static List<string> getProductIdsFromStringValues(String productCollectionValue) {
    List<string> productsIds = new ArrayList<>();
    String[] bundleProducts = productCollectionValue.split(Constants.COMMA_DELIMITER_REGEX);
    for (String bundleProduct : bundleProducts) {
      String[] bundleProductWithCount = bundleProduct.split(Constants.COUNT_SEPERATOR);
      if (bundleProductWithCount.length > 0) {
        productsIds.add(bundleProductWithCount[0]);
      }
    }
    return productsIds;
}

What I have tried:

my testcase is:

public void testgetProductIdsFromStringValues() throws Exception {
    String productCollectionValue = new String();
    List<string> productsIds = CatalogUtil.getProductIdsFromStringValues(productCollectionValue);
    Assert.assertTrue("List not empty", productsIds.isEmpty());
}

but it shows assertion error. I'm beginner in java and junit.anyhelp would be appreciated

Edit: Added from comment:

Assert.assertTrue("List not empty", productsIds.isEmpty());

this line shows error. because productIds returns "". so when I check ProductsIds.Empty it shows assertion error. but I need to pass productCollectionValue as a empty string, so only it fails in this if condition

if (bundleProductWithCount.length > 0) {
    productsIds.add(bundleProductWithCount[0]);
}