Initialise FCounterResponse members (#4084)

Initialising members to their default values prevents compiler warnings:

```
Int64Property FCounterResponse::Count is not initialized properly even though its struct probably has a custom default constructor. Module:Agones File:Classes/Classes.h
Int64Property FCounterResponse::Capacity is not initialized properly even though its struct probably has a custom default constructor. Module:Agones File:Classes/Classes.h
```

sdks/unreal/Agones/Source/Agones/Classes/Classes.h

@@ -423,10 +423,10 @@ struct FCounterResponse
 	}
 
 	UPROPERTY(BlueprintReadOnly, Category = "Agones")
-	int64 Count;
+	int64 Count = 0;
 
 	UPROPERTY(BlueprintReadOnly, Category = "Agones")
-	int64 Capacity;
+	int64 Capacity = 0;
 
 	explicit FCounterResponse(const TSharedPtr<FJsonObject> JsonObject)
 	{