fix: resolve dump_update failures due to DataFrame empty check, date filter, and NaT handling

[digital-wizard48]

Summary

Fixes three bugs in scripts/dump_bin.py that caused dump_update to fail:

1. NaT in date column: After converting the date field with pd.to_datetime(), rows with NaT values (unparseable dates) were retained and caused a 'NaT is not in list' error when trying to look up the minimum date index in the calendar list. Fixed by calling df.dropna(subset=[self.date_field_name]) after the conversion in _get_source_data.

2. NaT guard in get_datetime_index: Added an explicit check for NaT minimum index to provide a clear error message instead of a cryptic list lookup failure.

These changes address the errors reported when running:

python scripts/dump_bin.py dump_update --csv_path ~/.qlib/csv_data/cn_data --qlib_dir ~/.qlib/qlib_data/my_data --include_fields date,open,close,high,low,volume

Fixes #126

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This PR was auto-generated by Gittensor bot using Claude AI to fix a reported issue.