Fix zIndex handling for hitdetection

getZIndex returns number|undefined, so Number conversion is unnecessary, but
undefined was converted to NaN instead of zero.

src/ol/render/canvas/hitdetect.js

@@ -114,7 +114,7 @@ export function createHitDetectionImageData(
           })
         );
       }
-      const zIndex = Number(style.getZIndex());
+      const zIndex = style.getZIndex() || 0;
       let byGeometryType = featuresByZIndex[zIndex];
       if (!byGeometryType) {
         byGeometryType = {};