fix(media): don't 500 when the Unsplash/Giphy key is set to null

`config('services.unsplash.access_key', '')` only falls back to '' when the key
is *absent* — if the env var is present but empty, config returns null, and
assigning null to the `string` property throws a TypeError, taking the whole
request down with a 500 during media search.

Cast to `(string)` so a null/empty key degrades gracefully to '' and the service
returns an empty result set (as it already does for a missing key) instead of
throwing.

Covered by a regression test for both services.

app/Services/GiphyService.php

@@ -15,7 +15,7 @@ class GiphyService
 
     public function __construct()
     {
-        $this->apiKey = config('services.giphy.api_key', '');
+        $this->apiKey = (string) config('services.giphy.api_key', '');
     }
 
     /**

app/Services/UnsplashService.php

@@ -15,7 +15,7 @@ class UnsplashService
 
     public function __construct()
     {
-        $this->accessKey = config('services.unsplash.access_key', '');
+        $this->accessKey = (string) config('services.unsplash.access_key', '');
     }
 
     /**

tests/Unit/Services/MediaSearchKeyFallbackTest.php

@@ -0,0 +1,22 @@
+<?php
+
+declare(strict_types=1);
+
+use App\Services\GiphyService;
+use App\Services\UnsplashService;
+
+test('unsplash service tolerates a null access key without throwing', function () {
+    config(['services.unsplash.access_key' => null]);
+
+    $result = (new UnsplashService)->search('cats');
+
+    expect($result)->toBe(['results' => [], 'total' => 0, 'total_pages' => 0]);
+});
+
+test('giphy service tolerates a null api key without throwing', function () {
+    config(['services.giphy.api_key' => null]);
+
+    $result = (new GiphyService)->search('cats');
+
+    expect($result)->toBe(['results' => [], 'total' => 0, 'total_pages' => 0]);
+});