fix PHP 7 compatibility

Author: xabbuh

Message.php

@@ -124,11 +124,18 @@ public function toIterable(): iterable
 
     public function ensureValidity()
     {
-        if (!$this->headers->get('To')?->getBody() && !$this->headers->get('Cc')?->getBody() && !$this->headers->get('Bcc')?->getBody()) {
+        $to = (null !== $header = $this->headers->get('To')) ? $header->getBody() : null;
+        $cc = (null !== $header = $this->headers->get('Cc')) ? $header->getBody() : null;
+        $bcc = (null !== $header = $this->headers->get('Bcc')) ? $header->getBody() : null;
+
+        if (!$to && !$cc && !$bcc) {
             throw new LogicException('An email must have a "To", "Cc", or "Bcc" header.');
         }
 
-        if (!$this->headers->get('From')?->getBody() && !$this->headers->get('Sender')?->getBody()) {
+        $from = (null !== $header = $this->headers->get('From')) ? $header->getBody() : null;
+        $sender = (null !== $header = $this->headers->get('Sender')) ? $header->getBody() : null;
+
+        if (!$from && !$sender) {
             throw new LogicException('An email must have a "From" or a "Sender" header.');
         }