Do not require http_client service

This makes it possible to use AssetMapper with `http_client` disabled on the framework.

Example:
```php
$configurator->extension(
    'framework',
    [
        'http_client' => [
            'enabled' => false,
        ],
        'asset_mapper' => [
            'paths' => [
                'assets/',
            ],
        ],
    ]
);
```

Resources/config/asset_mapper.php

@@ -197,7 +197,7 @@
             ])
 
         ->set('asset_mapper.importmap.resolver', JsDelivrEsmResolver::class)
-            ->args([service('http_client')])
+            ->args([service('http_client')->nullOnInvalid()])
 
         ->set('asset_mapper.importmap.renderer', ImportMapRenderer::class)
             ->args([
@@ -212,12 +212,12 @@
         ->set('asset_mapper.importmap.auditor', ImportMapAuditor::class)
         ->args([
             service('asset_mapper.importmap.config_reader'),
-            service('http_client'),
+            service('http_client')->nullOnInvalid(),
         ])
         ->set('asset_mapper.importmap.update_checker', ImportMapUpdateChecker::class)
         ->args([
             service('asset_mapper.importmap.config_reader'),
-            service('http_client'),
+            service('http_client')->nullOnInvalid(),
         ])
 
         ->set('asset_mapper.importmap.command.require', ImportMapRequireCommand::class)