Fix issue in ValueMetricProducer where values are not reported

If a value metric has N values, not having a diff base will mean that N
calls of `onMatchedLogEventInternalLocked` are needed to output all values.

Bug: 123177418
Test: Unit tests `ValueMetricProducerTest#TestSkipZeroDiffOutput` amended
Test: Manual inspection of `cmd stats dump-report` shows all eight values
for CpuTimePerThreadFreq metrics
Change-Id: I8857c294ade049a4a42ef411d3d995610e04daf4