fix: make stdout/stderr optional

api.go

@@ -203,9 +203,13 @@ func hlrun(t *rt.Thread, c *rt.GoCont) (rt.Cont, error) {
 				stderrStream = f
 			}
 
-			strms = &streams{
-				stdout: stdoutStream.Handle(),
-				stderr: stderrStream.Handle(),
+			strms = &streams{}
+			if stdoutStream != nil {
+				strms.stdout = stdoutStream.Handle()
+			}
+
+			if stderrStream != nil {
+				strms.stderr = stderrStream.Handle()
 			}
 		} else {
 			if !terminalOut {

exec.go

@@ -272,11 +272,17 @@ func execCommand(cmd string, strms *streams) (io.Writer, io.Writer, error) {
 	runner, _ := interp.New()
 
 	if strms == nil {
-		strms = &streams{
-			stdout: os.Stdout,
-			stderr: os.Stderr,
+		strms = &streams{}
 	}
+
+	if strms.stdout == nil {
+		strms.stdout = os.Stdout
 	}
+
+	if strms.stderr == nil {
+		strms.stderr = os.Stderr
+	}
+
 	interp.StdIO(os.Stdin, strms.stdout, strms.stderr)(runner)
 
 	buf := new(bytes.Buffer)