[BUGFIX] Do not require receiver mail when receiver subject is empty

This fixes the flash message showing "Error, Mail could not sent correctly!"
when the receiver subject and the receiver mail is empty.

Resolves: in2code-de#611
Obsoletes: in2code-de#612

Author: cweiske

Classes/Domain/Service/Mail/SendMailService.php

@@ -87,14 +87,14 @@ public function sendMail(array $email, Mail $mail, array $settings, string $type
             $logger = ObjectUtility::getLogger(__CLASS__);
             $logger->info('Mail properties', [$email]);
         }
-        if (!GeneralUtility::validEmail($email['receiverEmail']) ||
-            !GeneralUtility::validEmail($email['senderEmail'])) {
-            return false;
-        }
         if (empty($email['subject'])) {
             // don't want an error flashmessage
             return true;
         }
+        if (!GeneralUtility::validEmail($email['receiverEmail']) ||
+            !GeneralUtility::validEmail($email['senderEmail'])) {
+            return false;
+        }
         return $this->prepareAndSend($email);
     }