fix: omit null color fields from filament/spool responses (fixes AFC lane colors)

[akira69]

Summary

Fixes two bugs that prevented AFC/BoxTurtle lane colors from rendering when FilaMan is used as a Spoolman drop-in via Moonraker's native [spoolman] integration.

Why

Bug 1 — null color fields break AFC lane coloring (router.py)

FastAPI serialises None optional fields as JSON nulls by default. Without response_model_exclude_none=True, a filament with no color returns:

{"color_hex": null, "multi_color_hexes": null, ...}

AFC does:

cur_lane.color = '#{}'.format(filament['color_hex'])

This produces "#None" — an invalid color string — for every filament with no color set, causing lane indicators to render blank.

Real Spoolman uses response_model_exclude_none=True on all spool/filament endpoints and simply omits null fields. Adding it here restores the same behavior.

Bug 2 — Empty string multi_color_hexes instead of None (service.py)

_filament_to_schema returned "" when a filament has no secondary colors. An empty string is not a valid absence-of-value marker — response_model_exclude_none only drops None, not "". Changed to None so the field is properly omitted.

Changes

router.py — add response_model_exclude_none=True to all filament and spool endpoints:

• GET /filament, GET /filament/{id}, POST /filament, PATCH /filament/{id}
• GET /spool, GET /spool/{id}, POST /spool, PATCH /spool/{id}, PUT /spool/{id}/use, PUT /spool/{id}/measure

service.py — _filament_to_schema: return None instead of "" for multi_color_hexes when no secondary colors are present.

Test Checklist

• GET /spoolman/api/v1/spool/{id} for a spool with color set: color_hex is present and contains hex without # (e.g. "2D6A3F")
• GET /spoolman/api/v1/spool/{id} for a spool without color set: color_hex field is absent from the response (not null)
• AFC/BoxTurtle lane colors render correctly for colored filaments
• AFC/BoxTurtle lane indicators do not show #None for uncolored filaments
• Moonraker [spoolman] integration initialises without errors

[akira69]

I cannot test this, just thought I'd take a swing at it with some AI agents - seems a reasonable proposal.