[7.13](backport #25428) Use working set bytes if usage bytes is zero

[mergify]

This is an automatic backport of pull request #25428 done by Mergify.

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[elasticmachine]

💚 Build Succeeded

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Build stats

• Build Cause: Pull request #25473 updated

• Start Time: 2021-05-03T16:05:40.736+0000

• Duration: 56 min 37 sec

• Commit: cd56993

Test stats 🧪

Test	Results
Failed	0
Passed	8308
Skipped	2260
Total	10568

Trends 🧪

💚 Flaky test report

Tests succeeded.

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Test stats 🧪

Test	Results
Failed	0
Passed	8308
Skipped	2260
Total	10568

[mergify]

This pull request is now in conflicts. Could you fix it? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/7.13/pr-25428 upstream/mergify/bp/7.13/pr-25428
git merge upstream/7.13
git push upstream mergify/bp/7.13/pr-25428