Homogenize wmts tileUrlFunction relative to other sources

src/ol/source/wmtssource.js

@@ -66,12 +66,6 @@ ol.source.WMTS = function(wmtsOptions) {
     goog.object.extend(kvpParams, context);
   }
 
-  var urls = wmtsOptions.urls;
-  if (!goog.isDef(urls)) {
-    goog.asserts.assert(goog.isDef(wmtsOptions.url));
-    urls = ol.TileUrlFunction.expandUrl(wmtsOptions.url);
-  }
-
   /**
    * @param {string} template Template.
    * @return {ol.TileUrlFunctionType} Tile URL function.
@@ -100,7 +94,13 @@ ol.source.WMTS = function(wmtsOptions) {
     };
   }
 
-  var tileUrlFunction = ol.TileUrlFunction.createFromTileUrlFunctions(
+  var tileUrlFunction = ol.TileUrlFunction.nullTileUrlFunction;
+  var urls = wmtsOptions.urls;
+  if (!goog.isDef(urls) && goog.isDef(wmtsOptions.url)) {
+    urls = ol.TileUrlFunction.expandUrl(wmtsOptions.url);
+  }
+  if (goog.isDef(urls)) {
+    tileUrlFunction = ol.TileUrlFunction.createFromTileUrlFunctions(
         goog.array.map(urls, function(url) {
           if (goog.isDef(kvpParams)) {
             // TODO: we may want to create our own appendParams function
@@ -110,6 +110,7 @@ ol.source.WMTS = function(wmtsOptions) {
           }
           return createFromWMTSTemplate(url);
         }));
+  }
 
   tileUrlFunction = ol.TileUrlFunction.withTileCoordTransform(
       function(tileCoord, tileGrid, projection) {