(standards): fix coding standards issue, with no trailing space after ! not operator.

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.");
           }