swarming: allow bot_overhead without isolate stats
swarming: allow bot_overhead without isolate stats

Bot has overhead even if it does not deal with isolate. In particular,
in future it will deal with cipd.

R=maruel@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2000273004

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