Added explanation about hashFiles returning an empty string (#15719)

Co-authored-by: Edward Thomson <ethomson@github.com>

content/actions/learn-github-actions/expressions.md

@@ -252,7 +252,7 @@ jobs:
 
 `hashFiles(path)`
 
-Returns a single hash for the set of files that matches the `path` pattern. You can provide a single `path` pattern or multiple `path` patterns separated by commas. The `path` is relative to the `GITHUB_WORKSPACE` directory and can only include files inside of the `GITHUB_WORKSPACE`. This function calculates an individual SHA-256 hash for each matched file, and then uses those hashes to calculate a final SHA-256 hash for the set of files. For more information about SHA-256, see "[SHA-2](https://en.wikipedia.org/wiki/SHA-2)."
+Returns a single hash for the set of files that matches the `path` pattern. You can provide a single `path` pattern or multiple `path` patterns separated by commas. The `path` is relative to the `GITHUB_WORKSPACE` directory and can only include files inside of the `GITHUB_WORKSPACE`. This function calculates an individual SHA-256 hash for each matched file, and then uses those hashes to calculate a final SHA-256 hash for the set of files. If the `path` pattern does not match any files, this returns an empty string. For more information about SHA-256, see "[SHA-2](https://en.wikipedia.org/wiki/SHA-2)."
 
 You can use pattern matching characters to match file names. Pattern matching is case-insensitive on Windows. For more information about supported pattern matching characters, see "[Workflow syntax for {% data variables.product.prodname_actions %}](/actions/using-workflows/workflow-syntax-for-github-actions/#filter-pattern-cheat-sheet)."