(standards): fix coding standards issue, add whitespace in fn.

Author: StyxOfDynamite

src/FcmMessage.php

@@ -80,7 +80,7 @@ public function condition(?string $condition): self
      */
     public function data(?array $data): self
     {
-        if (! empty(array_filter($data, fn($value) => ! is_string($value)))) {
+        if (! empty(array_filter($data, fn ($value) => ! is_string($value)))) {
             throw new \InvalidArgumentException('Data values must be strings.');
         }