Fix condition setting po number from input

Author: pmclain

app/code/Magento/QuoteGraphQl/Model/Resolver/SetPaymentMethodOnCart.php

@@ -70,7 +70,7 @@ public function resolve(Field $field, $context, ResolveInfo $info, array $value
         $paymentMethodCode = $args['input']['payment_method']['code'];
 
         $poNumber = isset($args['input']['payment_method']['purchase_order_number'])
-            && empty($args['input']['payment_method']['purchase_order_number'])
+            && !empty($args['input']['payment_method']['purchase_order_number'])
             ? $args['input']['payment_method']['purchase_order_number']
             : null;