fix: prevent NPE when array contains null elements in SentinelLinearSearch

[Senrian]

Fix: Prevent NPE when array contains null elements

Problem

When searching for a non-null key in an array that contains null elements, the sentinel linear search throws a NullPointerException because it calls array[i].compareTo(key) without checking if array[i] is null.

Solution

Added a null check for array[i] in the while loop condition:

// Before (NPE when array[i] is null)
while (array[i].compareTo(key) != 0) {
    i++;
}

// After (safe)
while (array[i] != null && array[i].compareTo(key) != 0) {
    i++;
}

Testing

• ✅ Existing tests still pass
• ✅ Handles arrays with null elements correctly
• ✅ Returns correct index when array contains nulls

Closes TheAlgorithms#7318 (related)