fix: volume viewport getCurrentImageId (#265)

* Fix issue when getting an index that equals 0.

When using this method while being on the first image, the index returns 0, and this method would return `undefined`.
This is unwanted since the first image obviously has a 0 index which is usable.
My fix simply consists of ensuring the returned value is a number (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN).

* Update VolumeViewport.ts

packages/core/src/RenderingEngine/VolumeViewport.ts

@@ -951,7 +951,7 @@ class VolumeViewport extends Viewport implements IVolumeViewport {
   public getCurrentImageId = (): string | undefined => {
     const index = this._getImageIdIndex();
 
-    if (!index) {
+    if (isNaN(index)) {
       return;
     }