fix: use commonConfig branchPrefix only if set

lib/workers/repository/index.js

@@ -85,7 +85,9 @@ async function renovateRepository(repoConfig, token, loop = 1) {
     return handleError(config, err);
   } finally {
     logger.setMeta({ repository: config.repository });
-    config.branchPrefix = commonConfig.branchPrefix;
+    config.branchPrefix = commonConfig
+      ? commonConfig.branchPrefix
+      : config.branchPrefix;
     await pruneStaleBranches(config);
     logger.info('Finished repository');
   }