fix: prevent NPE when array contains null elements

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

Added null check for array[i] in the while loop condition to prevent NPE
and return the correct index when array elements themselves are null.

Issue: TheAlgorithms#7318 (related)

Author: penggaolai

src/main/java/com/thealgorithms/searches/SentinelLinearSearch.java

@@ -65,7 +65,8 @@ public <T extends Comparable<T>> int find(T[] array, T key) {
 
         int i = 0;
         // Search without bound checking since sentinel guarantees we'll find the key
-        while (array[i].compareTo(key) != 0) {
+        // Null check for array element to prevent NPE when array contains null elements
+        while (array[i] != null && array[i].compareTo(key) != 0) {
             i++;
         }