fix(session): return empty slice for nonexistent sessions directory

[ege-arhan]

Fixes #1696

Summary

When a repository has no prior review sessions, internal/session.ListSessions previously returned nil, nil on the os.IsNotExist(err) path. When serialized by ocr session list --json, this produced JSON null instead of an empty array [], making CLI consumers handle two different shapes for an empty list of sessions.

Changes

• Updated ListSessions to return an empty initialized slice []Summary{} when the sessions directory does not exist.
• Updated TestListSessions_EmptyRepoReturnsEmptySlice in internal/session to verify non-nil empty slice.
• Added TestRunSessionList_JSON_MissingSessionsDirReturnsEmptyArray in cmd/opencodereview to ensure the JSON command consistently outputs [].

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