fix: handle Option<NaiveDate> when serializing end_date

[MackDing]

Fixes #65

p.end_date is Option<NaiveDate> so calling .to_string() directly fails to compile on stable Rust.

Verified with cargo check (0 errors).

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Note

Low Risk
Small, localized change to JSON formatting for a single field; main impact is that missing end_date now serializes as null instead of failing to compile.

Overview
Fixes JSON serialization for Position.end_date in print_positions by converting it with Option::map instead of calling .to_string() directly.

This makes end_date emit as either a string date or null in the JSON output and restores compilation on stable Rust when end_date is Option<NaiveDate>.

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