[BugFix] SecCompanyFilings: Purge nan Values & Make form_type Match

[deeleeramone]

1. Why?:

   • When attempting to filter for Form 4 resulted in other types of forms with '4' in it.
   • Unexpected nan values from empty dates.

2. What?:

   • Filters the form_type using "isin" instead of "contains".
   • Moves nan replace to the bottom, immediately before the output.

3. Impact:

   • Improved precision in the results.
   • Bug fix.

4. Testing Done:

# Multiple forms with '4' in it
obb.equity.fundamental.filings("CRM", form_type='4,144,S-4', limit=0, provider="sec")

# Just Form 4
obb.equity.fundamental.filings("CRM", form_type='4', limit=0, provider="sec").to_df().report_type.unique()