Query service fails without static assets

[yurishkuro]

This was introduced in #490.

I was debating it with @black-adder. The xdock was failing and the options were:

1. skip static handler if no static assets are found
2. create dummy index.html for xdock
3. introduce a new cmd line switch that explicitly disables static handler

Option (1) was the behavior before this change, but we've seen complaints about it before as a misconfiguration would go unnoticed until you try accessing the UI. We went with the simplest solution (2), but my preference is to do (3).

[pavolloffay]

I think there is also alternative to 3) simply disable static handler if --query.static-files is empty. By default it is set to jaeger-ui-build/whatever.

It is basically I have done in #493

[yurishkuro]

don't have a strong objection to that either, but it means all-in-one cannot be run with a simple command anymore

[pavolloffay]

don't have a strong objection to that either, but it means all-in-one cannot be run with a simple command anymore

Isn't that expected? (Option (1) was the behavior before this change, but we've seen complaints about it before as a misconfiguration would go unnoticed until you try accessing the UI.)

If a new flag is introduced it would require it too (to start without static assets).

[pavolloffay]

done in #493 -- Empty --query.static-files disables static handler.