AtlasTmxMapLoader small fixes

- Avoid NPE if atlas property is not set and throw gdx exception with error message instead
- removed unnecessary object creation


Caused by: java.lang.NullPointerException
	at com.badlogic.gdx.maps.tiled.AtlasTmxMapLoader.loadTileset(AtlasTmxMapLoader.java:340)
	at com.badlogic.gdx.maps.tiled.AtlasTmxMapLoader.loadMap(AtlasTmxMapLoader.java:266)
	at com.badlogic.gdx.maps.tiled.AtlasTmxMapLoader.loadAsync(AtlasTmxMapLoader.java:221)
	... 10 more