[BugFix] Fix Parsing Error In SEC Form13F

[deeleeramone]

1. Why?:

   • This fixes an issue in, obb.equity.ownership.form_13f, where aggregation silently failed because of unfilled NaN values in a string field.
   • Observable in when the 'asset_class' contained both options and share classes. Results would only aggregate the options positions.

2. What?:

   • Fills the NaN with a string value to allow the group aggregation, then replaces on output.
   • Renames a misnamed optional field to align with the definition, "voting_authority_other" -> "voting_authority_none"
   • General maintenance with some code cleanup.

3. Impact:

   • Fixes missing output
   • Slightly better performance

4. Testing Done:

   • This was generally focused around a very large file (25MB text) that takes a long time to process.
     • obb.equity.ownership.form_13f(symbol="0002012383", provider="sec")