Use an empty list for tags that there are too many
Use an empty list for tags that there are too many

This way the client-side UI knows the columns it should display, even
if there are too many values to keep track of.

BUG=631047

Review-Url: https://codereview.chromium.org/2360383002

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