Merged PR 55616: Revert "No response in REST client" update

## What's being changed

We're reverting / amending [this commit](669e9ae).

## Why it's being changed

In 4.25.2 we released a bugfix that was supposed to prevent a bad API response looking like a partial success, when connecting to a Dotdigital account. However, this change means that a number of V2 client methods that might return an empty array with a 200, (like `getContactsSuppressedSinceDate`) or a 204 No Content (like `deleteAddressBookContact`), are now dumping "No response in REST client" into the logs.

I've added an extra check into the account validation code to return false if the response is empty.

## How to review / test this change

- Go to your Dotdigital account screen in the Magento admin
- Set `$accountInfo` to null above L90 in Dotdigitalgroup\Email\Model\Apiconnector\Test
- Click to Save Config
- Confirm the account shows the expected 'Authorization has been denied' message

Related work items: #259561

Model/Apiconnector/Rest.php

@@ -293,8 +293,9 @@ public function execute()
 
         if (!$response) {
             $response = new stdClass();
-            $curlError = $this->getCurlError();
-            $response->message = $curlError ?: 'No response in REST client';
+            if ($curlError = $this->getCurlError()) {
+                $response->message = $curlError;
+            }
         }
 
         $this->responseMessage = $response->message ?? null;
@@ -643,11 +644,9 @@ public function setVerb($verb)
      */
     public function getCurlError()
     {
-        //if curl error
         if (!empty($this->curlError)) {
-            //log curl error
             $message = 'CURL ERROR ' . $this->curlError;
-            $this->helper->log($message);
+            $this->helper->error($message);
 
             return $this->curlError;
         }

Model/Apiconnector/Test.php

@@ -87,7 +87,7 @@ public function validate(string $apiUsername, string $apiPassword)
                 ->setApiPassword($apiPassword)
                 ->getAccountInfo($website->getId());
 
-            if (isset($accountInfo->message)) {
+            if (!$accountInfo || isset($accountInfo->message)) {
                 return false;
             }
             $scope = $website->getId() > 0 ? ScopeInterface::SCOPE_WEBSITES : ScopeConfigInterface::SCOPE_TYPE_DEFAULT;