Properly return null when there is no reply to address

Closes #6

Author: stevebauman

src/Message.php

@@ -124,31 +124,31 @@ public function subject(): ?string
      */
     public function from(): ?Address
     {
-        return head($this->addresses(HeaderConsts::FROM));
+        return head($this->addresses(HeaderConsts::FROM)) ?: null;
     }
 
     /**
      * Get the SENDER address.
      */
     public function sender(): ?Address
     {
-        return head($this->addresses(HeaderConsts::SENDER));
+        return head($this->addresses(HeaderConsts::SENDER)) ?: null;
     }
 
     /**
      * Get the REPLY-TO address.
      */
     public function replyTo(): ?Address
     {
-        return head($this->addresses(HeaderConsts::REPLY_TO));
+        return head($this->addresses(HeaderConsts::REPLY_TO)) ?: null;
     }
 
     /**
      * Get the IN-REPLY-TO address.
      */
     public function inReplyTo(): ?Address
     {
-        return head($this->addresses(HeaderConsts::IN_REPLY_TO));
+        return head($this->addresses(HeaderConsts::IN_REPLY_TO)) ?: null;
     }
 
     /**