fix(models): normalize empty-string trade and position icons to None

[cesarenaldi]

Sparse historical rows serialize missing market icons as "". The activity models already normalize this to None (PR #94); the loose Trade model and the Position/ClosedPosition models still passed "" through to the public icon fields.

This adds an icon before-validator using the existing empty_string_to_none helper on those three models, so missing icons consistently surface as None across the data surface.

Live evidence: 48/500 trades and 2/143 positions with icon: "" for a wallet with sparse historical rows.

Linear: DEV-264. Counterpart of ts-sdk PR #124 (DEV-263).

Verification

• New unit tests: empty icon parses to None on Trade, Position, and ClosedPosition; populated icons unchanged.
• uv run pytest tests/unit -q: 1701 passed.
• uv run ruff check ., uv run ruff format --check .: clean.
• uv run pyright: 0 errors.
• Live smoke: 500 trades (48 sparse) and 143 positions (2 sparse) parse with sparse icons as None.

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Note

Low Risk
Parsing-only normalization on optional display fields; behavior change is limited to "" → None with tests covering regressions.

Overview
Aligns missing market icon handling on the loose Trade model and Position / ClosedPosition with activity parsing: API rows that send icon: "" now surface as None instead of empty strings.

Each model gets a before validator on icon that delegates to the shared empty_string_to_none helper (same pattern as activity models after PR #94). Non-empty icon URLs are unchanged.

Unit tests cover empty vs populated icon on Trade, Position, and ClosedPosition.

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