channels: keep rendering carousel channels without an avatar

A missing avatar is a cosmetic gap, not a broken payload: raising on it
brings back the 500 this PR is meant to remove. The pageHeaderRenderer
branch already falls back to an empty string, so this matches it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

spec/invidious/channels/about_spec.cr

@@ -109,6 +109,29 @@ Spectator.describe "extract_auto_generated_channel_header" do
     end.to raise_error(InfoException)
   end
 
+  it "still renders a carousel channel that carries no avatar" do
+    initdata = JSON.parse(<<-JSON).as_h
+      {
+        "header": {
+          "carouselHeaderRenderer": {
+            "contents": [
+              {
+                "topicChannelDetailsRenderer": {
+                  "title": {"simpleText": "Sports"}
+                }
+              }
+            ]
+          }
+        }
+      }
+      JSON
+
+    header = extract_auto_generated_channel_header(initdata, "UCEgdi0XIXXZ-qJOFPf4JSKw")
+
+    expect(header[:author]).to eq("Sports")
+    expect(header[:author_thumbnail]).to eq("")
+  end
+
   it "parses the current pageHeaderRenderer shape" do
     # ex: https://www.youtube.com/channel/UCOpNcN46UbXVtpKMrmU4Abg (Gaming)
     initdata = JSON.parse(<<-JSON).as_h

src/invidious/channels/about.cr

@@ -82,8 +82,10 @@ def extract_auto_generated_channel_header(initdata : Hash(String, JSON::Any), uc
     author = details.dig?("title", "simpleText").try &.as_s
     author ||= raise InfoException.new("Could not extract the carousel title of channel #{ucid}")
     author_url = "https://www.youtube.com/channel/#{ucid}"
-    author_thumbnail = details.dig?("avatar", "thumbnails", 0, "url").try &.as_s
+
-    author_thumbnail ||= raise InfoException.new("Could not extract the carousel thumbnail of channel #{ucid}")
+    # A missing avatar is not worth failing the whole page over, and the
+    # `pageHeaderRenderer` branch above falls back to an empty string too.
+    author_thumbnail = details.dig?("avatar", "thumbnails", 0, "url").try &.as_s || ""
   else
     raise InfoException.new("Could not extract the header of channel #{ucid}")
   end