Check ol.source.UrlTile#urls property for null value

Fixes #4446

src/ol/source/tilearcgisrestsource.js

@@ -91,7 +91,7 @@ ol.source.TileArcGISRest.prototype.getRequestUrl_ =
         pixelRatio, projection, params) {
 
   var urls = this.urls;
-  if (urls.length === 0) {
+  if (!urls) {
     return undefined;
   }
 

src/ol/source/tilewmssource.js

@@ -212,7 +212,7 @@ ol.source.TileWMS.prototype.getRequestUrl_ =
         pixelRatio, projection, params) {
 
   var urls = this.urls;
-  if (urls.length === 0) {
+  if (!urls) {
     return undefined;
   }
 
@@ -298,10 +298,12 @@ ol.source.TileWMS.prototype.resetCoordKeyPrefix_ = function() {
   var i = 0;
   var res = [];
 
+  if (this.urls) {
     var j, jj;
     for (j = 0, jj = this.urls.length; j < jj; ++j) {
       res[i++] = this.urls[j];
     }
+  }
 
   var key;
   for (key in this.params_) {

test/spec/ol/source/tilewmssource.test.js

@@ -13,6 +13,16 @@ describe('ol.source.TileWMS', function() {
     };
   });
 
+  describe('constructor', function() {
+    it('can be constructed without url or urls params', function() {
+      var source = new ol.source.TileWMS({
+        projection: 'EPSG:3857',
+        tileGrid: ol.tilegrid.createXYZ({maxZoom: 6})
+      });
+      expect(source).to.be.an(ol.source.TileWMS);
+    });
+  });
+
   describe('#getTile', function() {
 
     it('returns a tile with the expected URL', function() {