Braintree - Skip generation Client Token if payment method is not active

For now Braintree module requests ClientToken via curl request
on every shopping cart and checkout page load.
Curl request takes 0.3-0.5 sec to respond for servers in N.Virginia

Author: andrey-legayev

app/code/Magento/Braintree/Model/Ui/ConfigProvider.php

@@ -67,11 +67,12 @@ public function __construct(
     public function getConfig()
     {
         $storeId = $this->session->getStoreId();
+        $isActive = $this->config->isActive($storeId);
         return [
             'payment' => [
                 self::CODE => [
-                    'isActive' => $this->config->isActive($storeId),
-                    'clientToken' => $this->getClientToken(),
+                    'isActive' => $isActive,
+                    'clientToken' => $isActive ? $this->getClientToken() : null,
                     'ccTypesMapper' => $this->config->getCcTypesMapper(),
                     'sdkUrl' => $this->config->getSdkUrl(),
                     'hostedFieldsSdkUrl' => $this->config->getHostedFieldsSdkUrl(),