Release unused file buffers for empty artifacts

internal/artifact/reader_benchmark_test.go

@@ -42,7 +42,20 @@ func BenchmarkArtifactFileRead(b *testing.B) {
 // BenchmarkRetainedArtifactFileRead measures live artifact bodies when an
 // opened file reports a size larger than the content that reaches EOF.
 func BenchmarkRetainedArtifactFileRead(b *testing.B) {
-	body := bytes.Repeat([]byte("x"), 256<<10)
+	for _, fixture := range []struct {
+		name string
+		body []byte
+	}{
+		{name: "small", body: bytes.Repeat([]byte("x"), 256<<10)},
+		{name: "empty", body: []byte{}},
+	} {
+		b.Run(fixture.name, func(b *testing.B) {
+			benchmarkRetainedArtifactFileRead(b, fixture.body)
+		})
+	}
+}
+
+func benchmarkRetainedArtifactFileRead(b *testing.B, body []byte) {
 	path := filepath.Join(b.TempDir(), "overreported.txt")
 	if err := os.WriteFile(path, body, 0o600); err != nil {
 		b.Fatal(err)