Handle callingPackageName being null properly in checkStartActivity().

callingPackageName can actually be null when started via command line,
etc. Previously the getApplicationInfo() call will fail if package
name is null and we'll ignore it so there won't be a crash, but it's
still better to handle it properly.

Bug: 131204827
Test: presubmit
Change-Id: I33ba43c28798226bed925940c2ffd09622372d09