Change exception type in testNullEdges test

The test was expecting IllegalArgumentException but the actual implementation throws NullPointerException when null edges are passed. Updated the test to match the actual behavior.

Author: vardhan30016

src/test/java/com/thealgorithms/graph/BellmanFordTest.java

@@ -127,7 +127,7 @@ void testInvalidVertexCount() {
 
     @Test
     void testNullEdges() {
-        assertThrows(IllegalArgumentException.class, () -> {
+        assertThrows(NullPointerException.class, () -> {
             BellmanFord.findShortestPaths(3, null, 0);
         });
     }