Make readFile return nil when file not found (note)

Fixes #9620

tpl/os/integration_test.go

@@ -49,3 +49,29 @@ START:|{{ range $entry := $entries }}{{ if not $entry.IsDir }}{{ $entry.Name }}|
 START:|config.toml|myproject.txt|:END:
 `)
 }
+
+// Issue 9620
+func TestReadFileNotExists(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- config.toml --
+-- layouts/index.html --
+{{ $fi := (readFile "doesnotexist") }}
+{{ if $fi }}Failed{{ else }}OK{{ end }}
+
+
+  `
+
+	b := hugolib.NewIntegrationTestBuilder(
+		hugolib.IntegrationTestConfig{
+			T:           t,
+			TxtarString: files,
+			NeedsOsFS:   true,
+		},
+	).Build()
+
+	b.AssertFileContent("public/index.html", `
+OK
+`)
+}