Dataset value is returned as undefined when it is set to empty string

[pdanpdan]

Describe the bug
Happy-Dom evaluates a dataset key as undefined when it is set to an empty string.
The browser evaluates it correctly to empty string.

To Reproduce
Steps to reproduce the behavior:

1. document.body.dataset.test = ''
2. console.log(document.body.dataset.test)
3. It will output undefined while it should output ''

Expected behavior
It should be evaluated to empty string.

Additional context
It is a regression introduced in v16.0.0 - latest working version is v15.11.7.

[pdanpdan]

A relevant regression test would be

element.setAttribute('data-test-empty', '');
const dataset = element.dataset;
expect(dataset.testEmpty).toBe('');
expect(dataset.nonExistentKey).toBeUndefined();
dataset.testEmptyAlso = '';
expect(dataset.testEmptyAlso).toBe('');