[SYS] Avoid transmission of MQTT password to the web page (#1878)

This commit avoid transmitting the saved MQTT user password to the web page for security reasons.
Instead we send the init password and check if it has been modified prior saving

main/main.ino

@@ -1791,7 +1791,7 @@ void setup_wifimanager(bool reset_settings) {
   WiFiManagerParameter custom_mqtt_server("server", "mqtt server", mqtt_server, parameters_size, " minlength='1' maxlength='64' required");
   WiFiManagerParameter custom_mqtt_port("port", "mqtt port", mqtt_port, 6, " minlength='1' maxlength='5' required");
   WiFiManagerParameter custom_mqtt_user("user", "mqtt user", mqtt_user, parameters_size, " maxlength='64'");
-  WiFiManagerParameter custom_mqtt_pass("pass", "mqtt pass", mqtt_pass, parameters_size, " input type='password' maxlength='64'");
+  WiFiManagerParameter custom_mqtt_pass("pass", "mqtt pass", MQTT_PASS, parameters_size, " input type='password' maxlength='64'");
   WiFiManagerParameter custom_mqtt_topic("topic", "mqtt base topic", mqtt_topic, mqtt_topic_max_size, " minlength='1' maxlength='64' required");
   WiFiManagerParameter custom_mqtt_secure("secure", "mqtt secure", "1", 2, mqtt_secure ? "type=\"checkbox\" checked" : "type=\"checkbox\"");
   WiFiManagerParameter custom_mqtt_cert("cert", "<br/>mqtt broker cert", mqtt_cert.c_str(), 4096);
@@ -1891,7 +1891,11 @@ void setup_wifimanager(bool reset_settings) {
     strcpy(mqtt_server, custom_mqtt_server.getValue());
     strcpy(mqtt_port, custom_mqtt_port.getValue());
     strcpy(mqtt_user, custom_mqtt_user.getValue());
-    strcpy(mqtt_pass, custom_mqtt_pass.getValue());
+    // Check if the MQTT password field contains the default value
+    if (strcmp(custom_mqtt_pass.getValue(), MQTT_PASS) != 0) {
+      // If it's not the default password, update the MQTT password
+      strcpy(mqtt_pass, custom_mqtt_pass.getValue());
+    }
     strcpy(mqtt_topic, custom_mqtt_topic.getValue());
     if (mqtt_topic[strlen(mqtt_topic) - 1] != '/' && strlen(mqtt_topic) < parameters_size) {
       strcat(mqtt_topic, "/");