toolbox/modprobe: Fix fallback path when mod_dirs is empty am: e067e96d am: b07ea76b

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/core/+/23452569



Change-Id: Icdab0cd14de49f96e3dd4052b4ecf7d248161d43
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>