Fix StartTLS mail setting.

application/src/main/java/org/thingsboard/server/service/mail/DefaultMailService.java

@@ -96,7 +96,7 @@ public class DefaultMailService implements MailService {
         javaMailProperties.put(MAIL_PROP + protocol + ".port", jsonConfig.get("smtpPort").asText());
         javaMailProperties.put(MAIL_PROP + protocol + ".timeout", jsonConfig.get("timeout").asText());
         javaMailProperties.put(MAIL_PROP + protocol + ".auth", String.valueOf(StringUtils.isNotEmpty(jsonConfig.get("username").asText())));
-        javaMailProperties.put(MAIL_PROP + protocol + ".starttls.enable", jsonConfig.get("enableTls"));
+        javaMailProperties.put(MAIL_PROP + protocol + ".starttls.enable", jsonConfig.has("enableTls") ? jsonConfig.get("enableTls").asText() : "false");
         return javaMailProperties;
     }