Updated Unit test coverage

Author: Nishitha0730

src/test/java/com/thealgorithms/matrix/MatrixMultiplicationTest.java

@@ -30,6 +30,19 @@ void testMultiply3by2and2by1(){
         assertMatrixEquals(expected, result);
     }
 
+    @Test
+    void testNullMatrixA() {
+        double[][] B = {{1, 2}, {3, 4}};
+        assertThrows(IllegalArgumentException.class, () -> MatrixMultiplication.multiply(null, B));
+    }
+
+    @Test
+    void testNullMatrixB() {
+        double[][] A = {{1, 2}, {3, 4}};
+        assertThrows(IllegalArgumentException.class, () -> MatrixMultiplication.multiply(A, null));
+    }
+
+
     @Test
     void testMultiplyNull(){
         double[][] matrixA = {{1.0,2.0},{3.0,4.0}};