chore(slogutil): ensure quoting of empty and confusing log values (#10236)

Clearer parsing (also for humans)

internal/slogutil/formatting.go

@@ -116,6 +116,7 @@ func expandAttrs(prefix string, a slog.Attr) []slog.Attr {
 }
 
 func appendAttr(sb *strings.Builder, prefix string, a slog.Attr, attrCount *int) {
+	const confusables = ` "()[]{},`
 	if a.Key == "" {
 		return
 	}
@@ -127,7 +128,7 @@ func appendAttr(sb *strings.Builder, prefix string, a slog.Attr, attrCount *int)
 	sb.WriteString(a.Key)
 	sb.WriteRune('=')
 	v := a.Value.Resolve().String()
-	if strings.ContainsAny(v, ` "()`) {
+	if v == "" || strings.ContainsAny(v, confusables) {
 		v = strconv.Quote(v)
 	}
 	sb.WriteString(v)