Have a look at Understanding Test Data and specifically Isolation of Test Data from Organization Data in Unit Tests:
Starting with Apex code saved using Salesforce API version 24.0 and later, test methods don’t have access by default to pre-existing data in the organization, such as standard objects, custom objects, and custom settings data, and can only access data that they create.
You have to create your own test data. A proper unit test setup cannot possibly work with a hard-coded Id. Basic structure of your test should look as follows:
static void testMyMethod()
SObject record = someSetupMethod();
// your setup method should insert to the database
List<Case> result = MyClass.myMethod(record.Id);
system.assertEquals(new List<Case>(), result, 'Include an informative message here');
Please note that even if your test would work with a hard-coded Id in this org, it wouldn't work when you try to deploy elsewhere. Never, ever use hard-coded Ids in your code base.