BDMS-468: Preserve legacy empty strings in NMA transfer pipelines

[jirhiker]

Why

This PR addresses the following problem / context:

• Preserve empty-string values from legacy sources during migration for specific NMA datasets.
• Ensure blank strings are not coerced into NULLs for legacy string fields.

How

Implementation summary - the following was changed / added / removed:

• Read targeted legacy CSVs with keep_default_na=False to keep empty strings.
• Added blank‑safe numeric/date parsing to map empty strings to NULL only for non-string fields.
• Added unit tests to prove "" is preserved for string fields in the specified transfers.

Notes

Any special considerations, workarounds, or follow-up work to note?

• Full test suite: source .venv/bin/activate && set -a && source .env && set +a && pytest

[Copilot]

Pull request overview

This PR modifies NMA transfer pipelines to preserve empty strings from legacy CSV sources instead of converting them to NULL values. The changes ensure that empty-string values in string fields are retained during migration while blank strings in numeric/date fields are still correctly converted to NULL.

Changes:

• Added keep_default_na=False parameter to CSV reading across multiple transfer modules
• Implemented is_blank() helper functions and blank-aware numeric/date parsing to differentiate between empty strings (preserved for string fields) and blanks that should become NULL for non-string fields
• Added comprehensive unit tests to verify empty string preservation for all affected transfer modules

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
transfers/weather_data.py	Added keep_default_na=False to CSV reading
transfers/surface_water_data.py	Added keep_default_na=False and blank-safe float/date parsing
transfers/radionuclides.py	Added keep_default_na=False and blank-safe numeric/date parsing
transfers/minor_trace_chemistry_transfer.py	Added keep_default_na=False and blank-safe float/date parsing
transfers/major_chemistry.py	Added keep_default_na=False and blank-safe numeric/date parsing
transfers/hydraulicsdata.py	Added keep_default_na=False and blank-safe numeric parsing with new as_float helper
transfers/chemistry_sampleinfo.py	Added keep_default_na=False and blank-safe date parsing
tests/test_transfer_preserve_empty_strings.py	New test file verifying empty string preservation across all transfer modules

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[chasetmartin]

Makes sense to me for all of these nma 1:1 transfers.