Update Email.php

Fix php stan error about creating TextPart with null instead of string - convert null to empty string if needed.

Signed-off-by: APaikens <shamanis@gmail.com>

Author: APaikens

src/Email.php

@@ -297,7 +297,7 @@ private function generateBody(): AbstractPart
             }
         }
 
-        return $part ?? new TextPart($this->text, $this->textCharset);
+        return $part ?? new TextPart((string) $this->text, $this->textCharset);
     }
 
     private function prepareParts(): array