Fix handling of empty /stdout result.
Fix handling of empty /stdout result.

If /stdout returns an empty dict (which happens if there's no stdout),
result['output'] becomes empty dict too, which later trips .rstrip(...) call.

BUG=781421
R=f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org

Change-Id: I9b0086b6686bdc5073fbe30452a2d72532119469
Reviewed-on: https://chromium-review.googlesource.com/754284
Reviewed-by: Stephen Martinis <f3fbe4c4218d2421400bf0bd07366865fd7a0c7e@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

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