Show message when My Friends feed is empty #773

Display helpful message when user hasn't followed anyone yet
on the My Friends page instead of showing empty content.

Changes:
- Add isEmpty check for 'my' tag in list-of-posts.tsx
- Add translation key empty_feed_not_following

Closes #773

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>