Return error when stdin provides no paths

When using "-" to read paths from stdin, if stdin is empty or contains
only blank lines, return an explicit error instead of silently succeeding
with no work done.

attrsum.go

@@ -99,8 +99,10 @@ func main() {
 // expandPaths expands the given paths, reading from stdin if "-" is present
 func expandPaths(args []string) ([]string, error) {
 	var paths []string
+	readFromStdin := false
 	for _, arg := range args {
 		if arg == "-" {
+			readFromStdin = true
 			scanner := bufio.NewScanner(os.Stdin)
 			for scanner.Scan() {
 				line := strings.TrimSpace(scanner.Text())
@@ -115,6 +117,12 @@ func expandPaths(args []string) ([]string, error) {
 			paths = append(paths, arg)
 		}
 	}
+	if len(paths) == 0 {
+		if readFromStdin {
+			return nil, errors.New("no paths provided on stdin")
+		}
+		return nil, errors.New("no paths provided")
+	}
 	return paths, nil
 }