fix(ConversationIcon): get rid of grey hairline, provide a fallback icon

[Antreesy]

☑️ Resolves

• Fix Avatars in conversation list sidebar have gray hairline #13324
  • See icon in TopBar for better reference
  • Also add failed state and fallback in case avatar wasn't loaded

🖌️ UI Checklist

🖼️ Screenshots / Screencasts

🏚️ Before	🏡 After
Failed state	--

🏁 Checklist

• 🌏 Tested with different browsers / clients:
  • Chromium (Chrome / Edge / Opera / Brave)
  • Firefox
  • Safari
  • Talk Desktop
  • Not risky to browser differences / client
• 🖌️ Design was reviewed, approved or inspired by the design team
• ⛑️ Tests are included or not possible
• 📗 User documentation in https://github.com/nextcloud/documentation/tree/master/user_manual/talk has been updated or is not required

[Antreesy]

/backport to stable31

[Antreesy]

/backport to stable30