Fix bulk-pantry-add crash on null unit (found in live MCP testing)

End-to-end testing (MCP client -> FastMCP -> Django) surfaced a 500:
adding an item with an explicit `unit: null` (as add_to_pantry sends
for items without a unit) hit a NOT NULL violation, because
`item_data.get("unit", "items")` returns None when the key is present.

- views.bulk_pantry_add: `item_data.get("unit") or "items"` — tolerate
  null/empty unit.
- mcp_server add_to_pantry: omit quantity/unit from the payload when
  unset, so the API applies its own defaults.
- test: bulk-add with unit/quantity = null returns 201 (25 pass).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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