Fix table sort issue

[lvybriage]

Fix issue #1764

If a trial haven't any metric, it will always in the final row when we sort these trials.

[liuzhe-lz]

What's the root cause of this problem?
Is a.accuracy or b.accuracy undefined when it's expected to have value?

[lvybriage]

What's the root cause of this problem?
Is a.accuracy or b.accuracy undefined when it's expected to have value?

a.accuracy means trial's final metric. but default metric column also has intermediate result.