Improve some null types for strictNullChecks

src/ol/source/Source.js

@@ -219,7 +219,7 @@ class Source extends BaseObject {
 /**
  * Turns the attributions option into an attributions function.
  * @param {AttributionLike|undefined} attributionLike The attribution option.
- * @return {?Attribution} An attribution function (or null).
+ * @return {Attribution|null} An attribution function (or null).
  */
 function adaptAttributions(attributionLike) {
   if (!attributionLike) {

src/ol/source/Vector.js

@@ -293,7 +293,7 @@ class VectorSource extends Source {
 
     /**
      * @private
-     * @type {Collection<import("../Feature.js").default<Geometry>>}
+     * @type {Collection<import("../Feature.js").default<Geometry>>|null}
      */
     this.featuresCollection_ = null;
 
@@ -683,7 +683,7 @@ class VectorSource extends Source {
    * Get the features collection associated with this source. Will be `null`
    * unless the source was configured with `useSpatialIndex` set to `false`, or
    * with an {@link module:ol/Collection} as `features`.
-   * @return {Collection<import("../Feature.js").default<Geometry>>} The collection of features.
+   * @return {Collection<import("../Feature.js").default<Geometry>>|null} The collection of features.
    * @api
    */
   getFeaturesCollection() {

src/ol/source/WMTS.js

@@ -367,7 +367,7 @@ export default WMTS;
  *  - format - {string} Image format for the layer. Default is the first
  *       format returned in the GetCapabilities response.
  *  - crossOrigin - {string|null|undefined} Cross origin. Default is `undefined`.
- * @return {?Options} WMTS source options object or `null` if the layer was not found.
+ * @return {Options|null} WMTS source options object or `null` if the layer was not found.
  * @api
  */
 export function optionsFromCapabilities(wmtsCap, config) {