Fix a bug in IntentFirewall.joinPackages

joinPackages returns null in most common cases - short package
names that do not exceed the sufficient length (125). In such cases,
the code will not fall into the else if branch inside the for loop. So
it will exit out of the for loop with sb not empty. This leads it
to execute the return null statement at the end of the method.

Bug: 444971007
Test: manual - check the log
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:e97993b2238ff33ff5a8061a4c4b92f30b1d9836
Merged-In: I25cf3265718ca8b84c953a03ae3aefaf84defdd7
Change-Id: I25cf3265718ca8b84c953a03ae3aefaf84defdd7