Add WATCH returns null standalone IT test

Signed-off-by: Andrew Carbonetto <andrew.carbonetto@improving.com>

java/integTest/src/test/java/glide/standalone/TransactionTests.java

@@ -8,6 +8,7 @@
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
@@ -185,4 +186,14 @@ public void zrank_zrevrank_withscores() {
         assertArrayEquals(new Object[] {0L, 1.0}, (Object[]) result[1]);
         assertArrayEquals(new Object[] {2L, 1.0}, (Object[]) result[2]);
     }
+
+    @Test
+    @SneakyThrows
+    public void WATCH_transaction_failure_returns_null() {
+        Transaction transaction = new Transaction();
+        transaction.get("key");
+        assertEquals(OK, client.customCommand(new String[] {"WATCH", "key"}).get());
+        assertEquals(OK, client.set("key", "foo").get());
+        assertNull(client.exec(transaction).get());
+    }
 }