Polish "Treat empty SSL bundle as unset"

Update Mail SSL configuration as well.

See gh-50624

Author: snicoll

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/mail/MailSenderPropertiesConfiguration.java

@@ -69,7 +69,7 @@ private void applyProperties(MailProperties properties, JavaMailSenderImpl sende
 		if (ssl.isEnabled()) {
 			javaMailProperties.setProperty("mail." + protocol + ".ssl.enable", "true");
 		}
-		if (ssl.getBundle() != null) {
+		if (StringUtils.hasLength(ssl.getBundle())) {
 			SslBundle sslBundle = sslBundles.getBundle(ssl.getBundle());
 			javaMailProperties.put("mail." + protocol + ".ssl.socketFactory",
 					sslBundle.createSslContext().getSocketFactory());

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/mail/MailSenderAutoConfigurationTests.java

@@ -245,6 +245,21 @@ void connectionOnStartupNotCalled() {
 			});
 	}
 
+	@Test
+	@WithPackageResources("test.jks")
+	void sslIsNotEnabledWhenBundleIsEmpty() {
+		this.contextRunner
+			.withPropertyValues("spring.mail.host:localhost", "spring.mail.ssl.bundle: ",
+					"spring.ssl.bundle.jks.test-bundle.keystore.location:classpath:test.jks",
+					"spring.ssl.bundle.jks.test-bundle.keystore.password:secret",
+					"spring.ssl.bundle.jks.test-bundle.key.password:password")
+			.run((context) -> {
+				assertThat(context).hasSingleBean(JavaMailSenderImpl.class);
+				JavaMailSenderImpl mailSender = context.getBean(JavaMailSenderImpl.class);
+				assertThat(mailSender.getJavaMailProperties().get("mail.smtp.ssl.socketFactory")).isNull();
+			});
+	}
+
 	@Test
 	void smtpSslEnabled() {
 		this.contextRunner.withPropertyValues("spring.mail.host:localhost", "spring.mail.ssl.enabled:true")