Ignore None dataset timestamps in Scene bounds

[sapunyangkut]

Summary

• ignore None dataset timestamps before calculating Scene.start_time and Scene.end_time
• fall back to reader times when every dataset timestamp is missing
• cover mixed valid/missing timestamps and all-missing fallback behavior

Closes #2883.

Validation

• focused regression cases: 3 passed on the exact upstream base and patch
• complete scene initialization module: 21 passed on the exact upstream base and patch
• adjacent configuration checks: 2 passed on the exact upstream base and patch
• git diff --check, reverse patch validation, and a bounded manual security review passed
• the current publication environment could not recollect pytest because pathlib_abc is not installed; no dependencies were installed for this submission

Checklist

• Closes Scene.start_time computation fails for datasets/composites with start_time=None (and same for end_time) #2883
• Tests added
• Added sapunyangkut to AUTHORS.md

AI assistance disclosure

This change was prepared and tested with OpenAI Codex. It is intentionally opened as a Draft for maintainer review. No automated issue/PR comments or Ready transition will be made.