fix(channel): harden About-enrich edge cases from review

- apply_channel_details: (branding.get("channel") or {}) so a "channel": null
  in brandingSettings can't AttributeError-abort the enrich batch.
- topicLabels: drop empty labels so a malformed topic URL can't render a blank chip.

frontend/src/components/ChannelPage.tsx

@@ -38,7 +38,8 @@ function topicLabels(urls: string[]): string[] {
     } catch {
       /* keep raw */
     }
-    seen.add(label.replace(/_/g, " "));
+    label = label.replace(/_/g, " ").trim();
+    if (label) seen.add(label);
   }
   return [...seen];
 }