Do not call pop_back when app has no uses-sdk

Calling pop_back on an empty container results in undefined behavior.
aapt2 dump badging seg faults when used on an APK with no uses-sdk.

Bug: 188461703
Test: aapt2 dump badging HelloAppTest.apk
Change-Id: I8d8308a06d542f003ae399629bdbfb5b845674fc