fix: detect empty cards when template uses special field conditionals

[marcusl07]

Linked issue (required)

Fixes #4750

Summary / motivation (required)

new_cards_required_normal() builds a set of non-empty fields from the note's actual fields, but only special-cases Tags. Other special fields like Deck, Subdeck, Card, CardFlag, Type, and CardID are always populated at render time but were missing from this set. This caused {{^Deck}} conditionals to be incorrectly evaluated as potentially rendering, making cards appear non-empty to the checker. The fix adds all always-present special fields to nonempty_fields.

Steps to reproduce (required)

1. Create a Basic (and reversed card) note with content in both fields
2. Change Card 1's front template to {{^Deck}}{{Back}}{{/Deck}}
3. Observe the previewer shows a blank card
4. Run Tools → Check → Empty Cards — no empty cards are reported

How to test (required)

Run ./ninja check:pytest — a new regression test test_empty_cards_special_fields in pylib/tests/test_cards.py covers this case.

Checklist (minimum)

• I ran ./ninja check or an equivalent relevant check locally.
• I added or updated tests when the change is non-trivial or behavior changed.

Details

New test verifies that a card with {{^Deck}}{{Back}}{{/Deck}} as its front template is correctly detected as empty.

Before / after behavior (optional)

Before: cards using special field conditionals like {{^Deck}} were not detected as empty. After: they are correctly flagged.

Risk / compatibility / migration (optional)

Low risk. Only affects empty card detection logic, not card rendering or scheduling.

UI evidence (required for visual changes; otherwise N/A)

N/A

Scope

• This PR is focused on one change (no unrelated edits).