Return immediately when matchmaking cannot pick an opponent, avoiding the misleading 'Will challenge None' log line.

Validation: python3 -m py_compile lichess-bot/lib/matchmaking.py

Type of pull request:

• Bug fix
• Feature
• Other

Description:

When random matchmaking finds no suitable online bot, choose_opponent() returns no username. The
caller already avoids creating a challenge in that case, but it still logs Will challenge None for a standard game.

This change returns immediately when no opponent was selected, clears the stored challenge id, and logs
that no challenge will be created.

Related Issues:

[Reference any related issues that this pull request addresses or closes. Use the syntax Closes #issue_number to automatically close the linked issue upon merging.]

Checklist:

• I have read and followed the contribution guidelines.
• I have added necessary documentation (if applicable).
• The changes pass all existing tests.

Screenshots/logs (if applicable):

ERROR    No suitable bots found to challenge.
INFO     Will challenge None for a standard game.

After:

ERROR    No suitable bots found to challenge.
INFO     No challenge will be created.