Handle dependencies with an empty manifest field

This happens sometimes with snapshots. We just want them to be displayed properly in the HTML output.

src/utils.ts

@@ -8,7 +8,9 @@ export function groupDependenciesByManifest(
 ): Map<string, Changes> {
   const dependencies: Map<string, Changes> = new Map()
   for (const change of changes) {
-    const manifestName = change.manifest
+    // If the manifest is null or empty, use a space as the key to avoid
+    // breaking the HTML rendering later
+    const manifestName = change.manifest || ' '
 
     if (dependencies.get(manifestName) === undefined) {
       dependencies.set(manifestName, [])