Refactor date header parsing logic

Improves the getParsedDateHeader method by restructuring the checks for DateHeader and its date value, ensuring null is returned for invalid cases and only converting valid dates.

Author: stevebauman

src/HasParsedMessage.php

@@ -32,11 +32,15 @@ public function date(): ?CarbonInterface
             return null;
         }
 
-        if ($header instanceof DateHeader) {
-            return Carbon::instance($header->getDateTime());
+        if (! $header instanceof DateHeader) {
+            return null;
+        }
+
+        if (! $date = $header->getDateTime()) {
+            return null;
         }
 
-        return null;
+        return Carbon::instance($date);
     }
 
     /**