fix: set cgiar-research-data api_url to null (#30)

cgspace.cgiar.org/rest 完全不可达（连接超时），
GARDIAN 平台 /api/v1/ 为 SPA 前端路由，无公开 REST API，
暂设为 null。

Author: firstdata-dev

firstdata/sources/international/agriculture/cgiar-research-data.json

@@ -10,7 +10,7 @@
   },
   "website": "https://www.cgiar.org",
   "data_url": "https://gardian.cgiar.org/",
-  "api_url": "https://cgspace.cgiar.org/rest",
+  "api_url": null,
   "country": null,
   "domains": [
     "agriculture",