Fix and strengthen tests in CollectionToCollectionConverterTests

[noojung]

Despite their names, convertEmptyVector_shouldReturnEmptyArrayList() and convertNonEmptyVector_shouldReturnNonEmptyArrayList() had identical logic:

	@Test
	void convertEmptyVector_shouldReturnEmptyArrayList() {
		Vector<String> vector = new Vector<>();
		vector.add("Element");
		testCollectionConversionToArrayList(vector);
	}

	@Test
	void convertNonEmptyVector_shouldReturnNonEmptyArrayList() {
		Vector<String> vector = new Vector<>();
		vector.add("Element");
		testCollectionConversionToArrayList(vector);
	}

As a result, the empty Vector case was never tested.

Update convertEmptyVector_shouldReturnEmptyArrayList() to use an empty Vector and assert that the result is an empty ArrayList. In convertNonEmptyVector_shouldReturnNonEmptyArrayList(), explicitly assert that the result is a non-empty ArrayList.

Also strengthen collectionsEmptyList() to verify that conversion returns the original instance, in addition to checking that it completes without an exception.