Fix opacity when the attribute is provided at page load, _opacity is

not set (setter not run), should return the attribute value if _opacity
is nullish.

src/layer.js

@@ -48,7 +48,8 @@ export class MapLayer extends HTMLElement {
   }
 
   get opacity() {
-    return this._opacity;
+    // use ?? since 0 is falsy, || would return rhs in that case
+    return this._opacity ?? this.getAttribute('opacity');
   }
 
   set opacity(val) {

test/e2e/layers/layerOpacityAttribute.test.js

@@ -17,33 +17,27 @@ test.describe('Adding Opacity Attribute to the Layer- Element', () => {
   });
 
   test('Setting Opacity Attibute to Layer- Element', async () => {
+    await page.pause();
     let opacity_attribute_value = await page.$eval(
       'body > mapml-viewer > layer-',
       (layer) => layer.getAttribute('opacity')
     );
-    if (!opacity_attribute_value) {
-      return;
-    } else {
-      let layer_opacity = await page.$eval(
-        'body > mapml-viewer > layer-',
-        (layer) => layer.opacity
-      );
-      expect(layer_opacity).toEqual(opacity_attribute_value);
-    }
+    let layer_opacity = await page.$eval(
+      'body > mapml-viewer > layer-',
+      (layer) => layer.opacity
+    );
+    expect(layer_opacity).toEqual(opacity_attribute_value);
   });
   test('Opacity Slider Value Test', async () => {
+    await page.pause();
     let opacity_slider_value = await page.$eval(
       'div > div.leaflet-control-container > div.leaflet-top.leaflet-right > div > section > div.leaflet-control-layers-overlays > fieldset > div:nth-child(2) > details > input[type=range]',
       (input) => input.value
     );
-    if (!opacity_slider_value) {
-      return;
-    } else {
-      let layer_opacity = await page.$eval(
-        'body > mapml-viewer > layer-',
-        (layer) => layer.opacity
-      );
-      expect(layer_opacity).toEqual(opacity_slider_value);
-    }
+    let layer_opacity = await page.$eval(
+      'body > mapml-viewer > layer-',
+      (layer) => layer.opacity
+    );
+    expect(layer_opacity).toEqual(opacity_slider_value);
   });
 });