What is the issue with the provided JUnit test case that tests the `isSorted()` method? The method simply checks if an array of ints is in ascending order. I am trying to thoroughly test the method but am not sure where to start. For example: check if array is empty or has only one element in it. Appreciate any help.

```
public class CheckArray {

    /**
     * Returns true if the given array is sorted in ascending order.
     */
    public static boolean isSorted(int[] a) {
        for (int i = 0; i < a.length - 1; i++) {
            if (a[i] > a[i + 1]) {
                return false;
            }
        }
        return true;
    }
}

public class CheckArrayTest {
    @Test
    public void isSorted() {
        int[] array = { 2, 1 };
        CheckArray haha = new CheckArray(array);
        assertFalse(haha.isSorted(null));
    }

```