Right now: a live stream with no thumbnail falls back to the streamer's avatar (an empty URL became openvibe.live's home page, drawn as a blank box) (#304)

ddiff --git a/server/home/activity.js b/server/home/activity.js
iindex a02be752..86635348 100644
---- a/server/home/activity.js
++++ b/server/home/activity.js
@@@ -31,7 +31,9 @@ let enabled = process.env.HOME_ACTIVITY !== 'off' && process.env.NODE_ENV !== 't
  let readers = {};
  
  const clip = (s, n) => { const t = String(s == null ? '' : s).replace(/\s+/g, ' ').trim(); return t.length > n ? `${t.slice(0, n - 1).trimEnd()}…` : t; };
--const abs = (base, u) => { try { const x = new URL(String(u || ''), base); return /^https:$/.test(x.protocol) ? x.href : null; } catch { return null; } };
++// An empty value is no URL (new URL('', base) is the base itself: a stream with no thumbnail drew openvibe.live's home
++// page as its picture, a blank box, instead of falling back to the streamer's avatar).
++const abs = (base, u) => { if (u == null || String(u).trim() === '') return null; try { const x = new URL(String(u), base); return /^https:$/.test(x.protocol) ? x.href : null; } catch { return null; } };
  const onHost = (u, host) => { try { const x = new URL(String(u || '')); return x.protocol === 'https:' && x.hostname === host ? x.href : null; } catch { return null; } };
  
  // The front door is the network's most public page, so it is stricter than the services it quotes: an item whose words
ddiff --git a/test/home-front-door.test.js b/test/home-front-door.test.js
iindex c02f8adc..12deed95 100644
---- a/test/home-front-door.test.js
++++ b/test/home-front-door.test.js
@@@ -20,6 +20,9 @@ assert.deepStrictEqual(live.map((s) => s.name), ['bob', 'Alice'], 'live and SFW
  assert.strictEqual(live[1].thumb, 'https://openvibe.live/api/thumbnails/a.jpg', 'a relative thumbnail becomes the public https URL');
  assert.strictEqual(live[0].thumb, null, 'a non-https thumbnail is dropped');
  assert.strictEqual(live[1].url, 'https://openvibe.live/@alice');
++// No thumbnail is no picture (it was openvibe.live's home page, drawn as a blank box): the card falls back to the avatar.
++const bare = activity.liveStreams({ streams: [{ is_live: 1, username: 'cara', title: 't', thumbnail_url: null, avatar_url: 'https://openvibe.media/o/med_1' }, { is_live: 1, username: 'dee', title: 't', thumbnail_url: '', avatar_url: '' }] });
++assert.deepStrictEqual(bare.map((s) => [s.thumb, s.avatar]), [[null, 'https://openvibe.media/o/med_1'], [null, null]]);
  
  const pastes = activity.publicPastes({ pastes: [
      { slug: 'one', title: 'Public one', visibility: 'public', type: 'code', language: 'js' },