fix: check for empty packageRules before applying

lib/util/package-rules.js

@@ -10,11 +10,12 @@ module.exports = {
 function applyPackageRules(inputConfig) {
   let config = { ...inputConfig };
   const { packageFile, depType, depName, currentVersion } = config;
+  const packageRules = config.packageRules || [];
   logger.trace(
-    { depName, packageRules: config.packageRules },
-    `Checking against ${config.packageRules.length} packageRules`
+    { depName, packageRules },
+    `Checking against ${packageRules.length} packageRules`
   );
-  config.packageRules.forEach(packageRule => {
+  packageRules.forEach(packageRule => {
     let {
       paths,
       depTypeList,