[Stats Refresh] Set empty chart bar height to chart height

[ScoutHarris]

Ref #11876

For empty charts, the grey bars are now the height of the chart.

To test:

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Latest Post Summary chart:

• The empty view on this one is a bit hard to trigger.
  • A site with views, but no views in the past 2 weeks.
  • A site with no views, but has Likes or Comments.
• Verify grey bars extend the height of the chart.

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Period Overview chart:

• On a site with no Views, Visitors, Likes, and/or Comments for all the periods on the chart.
• Verify grey bars extend the height of the chart.

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Post Stats chart:

• Access Post Stats for a site with no stats in the past 2 weeks.
• Verify grey bars extend the height of the chart.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[danielebogo]

@ScoutHarris it looks perfect!
It worked as expected!