[Minor] Fill remaining elements with null when provided array is longer

[tedyu]

In commit 42f267a, RangeSet#toArray() doesn't follow Java's practice of filling extra elements with null.
A sample program demonstrating Java's behavior can be found in the first comment.

This PR modifies toArray so that it follows Java's practice.

The testToArrayWithArrayParameter is modified to reflect the new behavior.

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[tedyu]

import java.util.*;
public class MyClass {
    public static void main(String args[]) {
        // Creating an empty Set
        Set<Integer> abs_col = new HashSet<Integer>();
 
        // Use add() method to add
        // elements into the Set
        abs_col.add(10);
        abs_col.add(15);
        abs_col.add(30);
        abs_col.add(20);
        abs_col.add(5);
        abs_col.add(25);
 
        // Displaying the Set
        System.out.println("The Set: " + abs_col);
 
        // Creating the array and using toArray()
        Integer[] arr = abs_col.toArray(new Integer[10]);
 
        System.out.println("The array is:");
        for (int j = 0; j < arr.length; j++)
            System.out.println(arr[j]);
    }
}

[tedyu]

@rreddy-22 @dajac
Can you review this PR ?