test: drop null input case flagged by Infer

Infer reports NULLPTR_DEREFERENCE for passing null to luhnCheck on purpose. The method does not document null handling, so the case is removed.

Author: datpp

src/test/java/com/thealgorithms/others/LuhnTest.java

@@ -2,7 +2,6 @@
 
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
-import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import org.junit.jupiter.api.Test;
@@ -103,9 +102,4 @@ void testInputArrayIsNotModified() {
         Luhn.luhnCheck(digits);
         assertArrayEquals(copy, digits);
     }
-
-    @Test
-    void testNullInputThrowsNullPointerException() {
-        assertThrows(NullPointerException.class, () -> Luhn.luhnCheck(null));
-    }
 }