fix(backend-github-graphql): return empty array on non existent folder (#3079)

When starting a new project, usually collection folders don't exist in the repo. This is a legitimate use case and not an error.

packages/netlify-cms-backend-github/src/GraphQLAPI.js

@@ -216,7 +216,7 @@ export default class GraphQLAPI extends API {
       const allFiles = this.getAllFiles(data.repository.object.entries, path);
       return allFiles;
     } else {
-      throw new APIError('Not Found', 404, 'GitHub');
+      return [];
     }
   }