fix: allow InsecureCredentialsWrapper::getAuthorizationHeaderCallback to return null (#541)

Author: ajupazhamayil

src/InsecureCredentialsWrapper.php

@@ -41,9 +41,13 @@ public function __construct()
     {
     }
 
-    public function getAuthorizationHeaderCallback($audience = null): callable
+    /**
+     * @param string $audience
+     * @return callable|null Returns null so the gRPC can accept it as an insecure channel.
+     */
+    public function getAuthorizationHeaderCallback($audience = null): ?callable
     {
-        return fn() => [];
+        return null;
     }
 
     public function checkUniverseDomain(): void

src/Transport/HttpUnaryTransportTrait.php

@@ -109,7 +109,8 @@ private static function buildCommonHeaders(array $options)
             // Prevent unexpected behavior, as the authorization header callback
             // uses lowercase "authorization"
             unset($headers['authorization']);
-            $authHeaders = $callback();
+            // Mitigate scenario where InsecureCredentialsWrapper returns null.
+            $authHeaders = empty($callback) ? [] : $callback();
             if (!is_array($authHeaders)) {
                 throw new \UnexpectedValueException(
                     'Expected array response from authorization header callback'