fix: abort if renovate config returns null

lib/workers/repository/init/config.js

@@ -58,6 +58,11 @@ async function mergeRenovateConfig(config) {
     logger.info({ config: renovateJson }, 'package.json>renovate config');
   } else {
     const renovateConfig = await platform.getFile(configFile);
+    // istanbul ignore if
+    if (!renovateConfig) {
+      logger.warn('Fetching renovate config returns null');
+      throw new Error('registry-failure');
+    }
     let allowDuplicateKeys = true;
     let jsonValidationError = jsonValidator.validate(
       renovateConfig,