Fix semver compare when first version

[oney]

In first version(not install any update from CodePush), we will get [CodePush] An unknown error occurred. after calling CodePush.sync();.
In CodePush.js line 55

console.log('localPackage', localPackage); // -> { failedInstall: false, isFirstRun: false }

if (localPackage && semver.compare(localPackage.appVersion, config.appVersion) === 0) {
 queryPackage = localPackage;
}

Because localPackage.appVersion is undefined, semver.compare will throw an exception.

try {
  semver.compare(localPackage.appVersion, config.appVersion);
} catch (e) {
  console.log('This is an error', e);
}

Logs are

'This is an error', { [TypeError: Invalid Version: undefined]
  line: 57942,
  column: 20,
  sourceURL: 'http://localhost:8081/index.ios.bundle?platform=ios&dev=true' }

So, just check localPackage.appVersion first then compare them.

[lostintangent]

LGTM! I obviously forgot to test this change in an app with no previous updates. Thanks for this contribution @oney!