add default value for globPatterns

without this, the latest alpha broke on start
```
/Users/adeviankakrisnafadlil/Projects/eurecah/node_modules/react-scripts/config/paths.js:130
    .reduce(
     ^

TypeError: Cannot read property 'reduce' of undefined
    at findPkgs (/Users/adeviankakrisnafadlil/Projects/eurecah/node_modules/react-scripts/config/paths.js:130:6)
    at getMonorepoPkgPaths (/Users/adeviankakrisnafadlil/Projects/eurecah/node_modules/react-scripts/config/paths.js:143:22)
    at Object.<anonymous> (/Users/adeviankakrisnafadlil/Projects/eurecah/node_modules/react-scripts/config/paths.js:155:55)
    at Module._compile (module.js:643:30)
    at Object.Module._extensions..js (module.js:654:10)
    at Module.load (module.js:556:32)
    at tryModuleLoad (module.js:499:12)
    at Function.Module._load (module.js:491:3)
    at Module.require (module.js:587:17)
    at require (internal/module.js:11:18)
error Command failed with exit code 1.
```

packages/react-scripts/config/paths.js

@@ -120,7 +120,7 @@ if (useTemplate) {
 
 module.exports.srcPaths = [module.exports.appSrc];
 
-const findPkgs = (rootPath, globPatterns) => {
+const findPkgs = (rootPath, globPatterns = []) => {
   const globOpts = {
     cwd: rootPath,
     strict: true,