GetFallbackFont() called .First() on the font file sequence, which throws
System.InvalidOperationException when no file matched the requested name,
causing HTTP 500. Change to .FirstOrDefault() so a missing font falls
through to the existing null guard and returns HTTP 200 OK (the empty
response is intentional to avoid breaking SubtitlesOctopus).
Fixes#17683.
Prepared with AI assistance (Claude Code, Anthropic), reviewed for correctness before submission.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
zerafachris
20678c9a8c