adds empty param constructor

WiFiManager.cpp

@@ -22,6 +22,10 @@ uint8_t WiFiManager::_lastconxresulttmp = WL_IDLE_STATUS;
  * --------------------------------------------------------------------------------
 **/
 
+WiFiManagerParameter::WiFiManagerParameter() {
+  WiFiManagerParameter("");
+}
+
 WiFiManagerParameter::WiFiManagerParameter(const char *custom) {
   _id             = NULL;
   _label          = NULL;

WiFiManager.h

@@ -104,6 +104,7 @@ class WiFiManagerParameter {
         Create custom parameters that can be added to the WiFiManager setup web page
         @id is used for HTTP queries and must not contain spaces nor other special characters
     */
+    WiFiManagerParameter();
     WiFiManagerParameter(const char *custom);
     WiFiManagerParameter(const char *id, const char *label);
     WiFiManagerParameter(const char *id, const char *label, const char *defaultValue, int length);

examples/Advanced/Advanced.ino

@@ -7,7 +7,7 @@
 #define TRIGGER_PIN 0
 
 WiFiManager wm; // global wm instance
-WiFiManagerParameter custom_field(""); // global param ( for non blocking w params )
+WiFiManagerParameter custom_field; // global param ( for non blocking w params )
 
 void setup() {
   Serial.begin(115200);