Only zip2zip bundle resources if they exist

Only attempt to zip2zip the bundle parts/res.zip if it was created
because of full_classes_jar or PRIVATE_EXTRA_JAR_ARGS.

Bug: 117295826
Test: m dist on unbundled target with no resources or classes
Change-Id: I6072b29c7331201f33d196c24154f9947b4b0053