Added empty alert response support

It was a wrong decision to throw an exception for this situation, because it's *expected* + it happens 99.99% of the time - which caused #onFailedRequest spam.

Author: DavidTheExplorer

src/main/java/dte/tzevaadomapi/alertsource/AlertSource.java

@@ -6,4 +6,7 @@
 public interface AlertSource
 {
 	Alert getMostRecentAlert() throws Exception;
+	
+	
+	public static final Alert EMPTY_RESPONSE = null;
 }

src/main/java/dte/tzevaadomapi/alertsource/json/JSONAlertSource.java

@@ -31,7 +31,7 @@ public Alert getMostRecentAlert() throws Exception
 		JSONArray alertsJsonArray = requestAlertsJSON();
 
 		if(alertsJsonArray == null || alertsJsonArray.isEmpty())
-			throw new IllegalArgumentException("Cannot get the most recent alert due to an empty JSON response!");
+			return EMPTY_RESPONSE;
 
 		return fromJSON((JSONObject) alertsJsonArray.get(0));
 	}

src/main/java/dte/tzevaadomapi/notifier/TzevaAdomNotifier.java

@@ -56,7 +56,11 @@ public CompletableFuture<Void> listen()
 			{
 				Alert alert = getMostRecentAlert();
 
-				//if the last alert in history doesn't equal to the last requested one - It's TZEVA ADOM
+				//ignore empty responses
+				if(alert == AlertSource.EMPTY_RESPONSE)
+					continue;
+				
+				//if the last alert in history equals the the last requested - it's not Tzeva Adom
 				if(lastTzevaAdom.equals(alert)) 
 					continue;