Fix -1 issue, probably fixing issues: #17, #25, #38 (point nr 3) and #43

 (#52)

Values used in the code are -1, if you use NAN home assistant will understand it's a wrong value

Probably fixing issues: #17, #25, #38 (point nr 3) and #43 (#52)

esphome-opentherm/opentherm_component.h

@@ -60,17 +60,17 @@ class OpenthermComponent: public PollingComponent {
   }
   float getExternalTemperature() {
       unsigned long response = ot.sendRequest(ot.buildRequest(OpenThermRequestType::READ, OpenThermMessageID::Toutside, 0));
-      return ot.isValidResponse(response) ? ot.getFloat(response) : -1;
+      return ot.isValidResponse(response) ? ot.getFloat(response) : NAN;
   }
 
   float getReturnTemperature() {
       unsigned long response = ot.sendRequest(ot.buildRequest(OpenThermRequestType::READ, OpenThermMessageID::Tret, 0));
-      return ot.isValidResponse(response) ? ot.getFloat(response) : -1;
+      return ot.isValidResponse(response) ? ot.getFloat(response) : NAN;
   }
 
   float getHotWaterTemperature() {
       unsigned long response = ot.sendRequest(ot.buildRequest(OpenThermRequestType::READ, OpenThermMessageID::Tdhw, 0));
-      return ot.isValidResponse(response) ? ot.getFloat(response) : -1;
+      return ot.isValidResponse(response) ? ot.getFloat(response) : NAN;
   }
 
   bool setHotWaterTemperature(float temperature) {
@@ -82,12 +82,12 @@ class OpenthermComponent: public PollingComponent {
 
   float getModulation() {
     unsigned long response = ot.sendRequest(ot.buildRequest(OpenThermRequestType::READ, OpenThermMessageID::RelModLevel, 0));
-    return ot.isValidResponse(response) ? ot.getFloat(response) : -1;
+    return ot.isValidResponse(response) ? ot.getFloat(response) : NAN;
   }
 
   float getPressure() {
     unsigned long response = ot.sendRequest(ot.buildRequest(OpenThermRequestType::READ, OpenThermMessageID::CHPressure, 0));
-    return ot.isValidResponse(response) ? ot.getFloat(response) : -1;
+    return ot.isValidResponse(response) ? ot.getFloat(response) : NAN;
   }
 
   void update() override {
@@ -161,4 +161,4 @@ class OpenthermComponent: public PollingComponent {
     heatingWaterClimate->publish_state();
   }
 
-};
+};