soong_zip after dex should not fail when no *.dex files were generated

This can happen if Android application has only a manifest file and no
java code.

Fixes: 124483155
Test: treehugger with Android.mk converted to Android.bp in
platforms/frameworks/base/tests/privapp-permnissions

Change-Id: I45f95680b62a7a4bda0e7ab156f3df24b0dad70b