Fix crash when no browsers are installed

[NHellFire]

When no browsers are installed, it will currently crash with:

System.err: java.lang.NullPointerException: Attempt to read from field 'android.content.pm.ActivityInfo android.content.pm.ResolveInfo.activityInfo' on a null object reference
System.err:   at jp.co.cyberagent.stf.util.BrowserUtil.filterChooser(BrowserUtil.java:43)
System.err:   at jp.co.cyberagent.stf.util.BrowserUtil.getDefaultBrowser(BrowserUtil.java:37)
System.err:   at jp.co.cyberagent.stf.monitor.BrowserPackageMonitor.report(BrowserPackageMonitor.java:87)
System.err:   at jp.co.cyberagent.stf.monitor.BrowserPackageMonitor.peek(BrowserPackageMonitor.java:77)
System.err:   at jp.co.cyberagent.stf.Service$Server$Connection.run(Service.java:371)
System.err:   at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:428)
System.err:   at java.util.concurrent.FutureTask.run(FutureTask.java:237)
System.err:   at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
System.err:   at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
System.err:   at java.lang.Thread.run(Thread.java:761)