Fixed empty timestamp in Home-Assistant JSON (#1105)

* Nullable timestamps for HA JSON

* Nullable timestamps for MQTT JSON

lib/HomeAssistantMqttHandler/json/ha1.json

@@ -1,4 +1,4 @@
 {
     "P" : %lu,
-    "t" : "%s"
+    "t" : %s
 }

lib/HomeAssistantMqttHandler/json/ha2.json

@@ -3,6 +3,6 @@
     "tPO" : %.3f,
     "tQI" : %.3f,
     "tQO" : %.3f,
-    "rtc" : "%s",
-    "t" : "%s"
+    "rtc" : %s,
+    "t" : %s
 }

lib/HomeAssistantMqttHandler/json/ha3.json

@@ -12,5 +12,5 @@
     "U1" : %.2f,
     "U2" : %.2f,
     "U3" : %.2f,
-    "t" : "%s"
+    "t" : %s
 }

lib/HomeAssistantMqttHandler/json/ha4.json

@@ -28,5 +28,5 @@
     "tPO1" : %.3f,
     "tPO2" : %.3f,
     "tPO3" : %.3f,
-    "t" : "%s"
+    "t" : %s
 }