fix(01-review): WR-07 base64ToBlob empty-input shortcut + SW-side empty-segment filter

src/shared/binary.ts

@@ -65,6 +65,17 @@ export async function blobToBase64(blob: Blob): Promise<string> {
  * @returns A Blob whose bytes match the original encoded blob exactly.
  */
 export function base64ToBlob(b64: string, mimeType: string): Blob {
+  // WR-07 fix: defensive early-return for empty input. `atob('')` returns
+  // an empty string and the resulting Blob has size 0 — currently no
+  // caller filters zero-size segments, which would corrupt the
+  // concatenated WebM with a stray empty EBML segment. Return an
+  // explicit empty Blob here so the SW-side filter on
+  // `segment.data.size > 0` (see src/background/index.ts
+  // getVideoBufferFromOffscreen) can pre-filter cleanly without an
+  // extra atob round-trip.
+  if (b64.length === 0) {
+    return new Blob([], { type: mimeType });
+  }
   const binary = atob(b64);
   const bytes = new Uint8Array(binary.length);
   for (let i = 0; i < binary.length; i++) {