Fix `logdog_cat` listing project namespace root.
Fix `logdog_cat` listing project namespace root.

Currently, "ls" with no parameters errors saying that a project must be
specified. This CL makes it so that no parameters shows the virtual
project-level root, which was the intended behavior.

BUG=None
TEST=local
  - Ran it and it worked.

R=nodir@chromium.org

Review-Url: https://codereview.chromium.org/2189543003

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go