fix polling null from the available pooled connections

Signed-off-by: Billy Yuan <billy112487983@gmail.com>

Author: BillyYccc

vertx-sql-client/src/main/java/io/vertx/sqlclient/impl/pool/ConnectionPool.java

@@ -242,6 +242,10 @@ private void check() {
         while (waiters.size() > 0) {
           if (available.size() > 0) {
             PooledConnection proxy = available.poll();
+            if (proxy == null) {
+              // available is empty?
+              return;
+            }
             Handler<AsyncResult<Connection>> waiter = waiters.poll();
             waiter.handle(Future.succeededFuture(proxy));
           } else {