client: handle None from _get_recursive_size in local_caching.py
client: handle None from _get_recursive_size in local_caching.py

This is followup for
https://crrev.com/c/2230747/7/client/local_caching.py#947

Change-Id: Ic0090a2020d16d401feab75d090d8ebdaccd82ab
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2376107
Commit-Queue: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Reviewed-by: Ye Kuang <1cd89f22f3967fc3dcb883b53940be4b98ec483f@google.com>
Auto-Submit: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py