fix Message::bodySummary when preg_replace is broken

preg_replace returns "false" on failure which interpreted as OK result which is wrong.

Reform statement in positive manner makes it right

src/Message.php

@@ -77,11 +77,11 @@ public static function bodySummary(MessageInterface $message, int $truncateAt =
 
         // Matches any printable character, including unicode characters:
         // letters, marks, numbers, punctuation, spacing, and separators.
-        if (preg_match('/[^\pL\pM\pN\pP\pS\pZ\n\r\t]/u', $summary)) {
-            return null;
+        if (preg_match('/[\pL\pM\pN\pP\pS\pZ\n\r\t]+/u', $summary)) {
+            return $summary;
         }
 
-        return $summary;
+        return null;
     }
 
     /**