Added explanation about hashFiles returning an empty string

[nikola-jokic]

Why:

Closes #15717, Relates to actions/runner issue

What's being changed:

I just added one sentence explaining that if the path does not match any known path, an empty string would be returned

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[ramyaparimi]

@nikola-jokic
Thanks so much for opening a PR! I'll get this triaged for review ⚡

[github-actions]

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Content directory changes

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Source	Preview	Production	What Changed
content/actions/learn-github-actions/expressions.md	Modified	Original

[ethomson]

Great. Thanks for digging in to the https://github.com/actions/runner repository and turning that into really meaningful improvements. This is great stuff @nikola-jokic

[skedwards88]

Thanks for this PR! I just made one minor wording suggestion. I'll go ahead and incorporate that, then we'll get this merged down for you.

[github-actions]

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