Let get-prebuilt-src-arch return empty if the input is empty

The list of archs of a pure java apk (no native libs) is empty.
In this case get-prebuilt-src-arch still return 'none'.
If it's set to LOCAL_MODULE_TARGET_ARCH, it makes the apk
be incompatible with the building target. It's not good.

Just return empty in this case.

Change-Id: Ide75cdb08c8480c8e7d3a0ac59f41831fbc18bd5