fix: assert file system path for skopeo copy is truthy

pullImages implicitly relies on the list of pullable images it receives to have fileSystemPaths for each image, as a valid path in the filesystem where we can instruct Skopeo to pull images to.
right now, it accepts undefined values as these paths, resulting in calls to Skopeo creating a file named "undefined" on the filesystem.
this commit amends this behaviour by asserting the fileSystemPath is truthy before calling Skopeo.

Author: Shesekino

src/scanner/images/index.ts

@@ -8,6 +8,10 @@ export async function pullImages(images: IPullableImage[]): Promise<IPullableIma
 
   for (const image of images) {
     const {imageName, fileSystemPath} = image;
+    if (!fileSystemPath) {
+      continue;
+    }
+
     try {
       await skopeoCopy(imageName, fileSystemPath);
       pulledImages.push(image);