Captcha.setBackground为null时背景透明

hutool-swing/src/test/java/org/dromara/hutool/swing/captcha/CaptchaTest.java

@@ -44,6 +44,15 @@ public class CaptchaTest {
 		lineCaptcha.write("f:/test/captcha/tellow.png");
 	}
 
+	@Test
+	@Disabled
+	public void noBackgroundCaptchaTest() {
+		// 定义图形验证码的长和宽
+		final LineCaptcha lineCaptcha = CaptchaUtil.ofLineCaptcha(200, 70, 4, 15);
+		lineCaptcha.setBackground(null);
+		lineCaptcha.write("d:/test/captcha/tellow.png");
+	}
+
 	@Test
 	@Disabled
 	public void lineCaptchaWithMathTest() {