HistogramTester::GetHistogramSamplesSinceCreation never returns null
Currently, GetHistogramSamplesSinceCreation can return null if no histogram was generated. But that depends also on tests run prior to the current test. This CL changes this to returning empty samples instead, so that independently of previously run tests, the current test always gets the same response even if it does not generate any histograms.
BUG=473689
Review URL: https://codereview.chromium.org/
1264123002
Cr-Commit-Position: refs/heads/master@{#341498}