cmd/tailscale/cli: disallow empty text "" from serve CLI (#6416)

Current behavior is broken. tailscale serve text / "" returns no error
and shows up in tailscale serve status but requests return a 500
"empty handler".

Adds an error if the user passes in an empty string for the text
handler.

Closes #6405

Signed-off-by: Shayne Sweeney <shayne@tailscale.com>

cmd/tailscale/cli/serve.go

@@ -285,6 +285,9 @@ func (e *serveEnv) runServe(ctx context.Context, args []string) error {
 		}
 		h.Proxy = t
 	case "text":
+		if args[2] == "" {
+			return errors.New("unable to serve; text cannot be an empty string")
+		}
 		h.Text = args[2]
 	default:
 		fmt.Fprintf(os.Stderr, "error: unknown serve type %q\n\n", args[1])