Tracing - Check for undefined histograms input in histogram-set-view.
Tracing - Check for undefined histograms input in histogram-set-view.

The histograms param is undefined in explicitUndefinedHistogramSet, and
the intention in the code looks like it was to display "zero Histograms" the
same way an empty HistogramSet() does. Added a check and early out in the
same spot.

BUG=catapult:#484

Review-Url: https://codereview.chromium.org/2814243003

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