Serve WMS tiles for sources without specified extent

With this change, WMS sources do not need an extent configured.
The result is that the WMS will be queried for tiles for any
extent.

src/ol/source/tiledwmssource.js

@@ -64,7 +64,7 @@ ol.source.TiledWMS = function(options) {
       tileExtent = tileGrid.getTileCoordExtent(
           new ol.TileCoord(tileCoord.z, x, tileCoord.y));
     }
-    if (!ol.extent.intersects(tileExtent, extent)) {
+    if (!goog.isNull(extent) && !ol.extent.intersects(tileExtent, extent)) {
       return null;
     }
     return new ol.TileCoord(tileCoord.z, x, tileCoord.y);