Fix #2943: preserve falsy formula results (0, false, empty string) in copy

[senoff]

Summary

Fixes exceljs/exceljs#2943: copying a formula cell whose cached result is 0, false, or '' produced a destination cell with no result.

The root cause is FormulaValue._copyModel in lib/doc/cell.js, which iterates over candidate fields with if (value) — a truthy check that drops every falsy result. Because _copyModel backs both the value getter and setter, the bug fires on the canonical copy idiom dest.value = src.value (and on every plain read of cell.value on a formula cell with a falsy cached result).

While auditing the same file I found three sibling falsy-check bugs and one xlsx-reader bug with the same shape, all fixed in this PR.

Changes

lib/doc/cell.js — FormulaValue

• _copyModel: replace if (value) with if (name in model && model[name] !== undefined) so 0, false, '' survive the copy.
• toCsvString: replace ${this.model.result || ''} with an explicit null/undefined check; previously result === 0 rendered as empty string in CSV.
• toString: replace this.model.result ? ... : '' ternary with the same explicit check.
• effectiveType: add a typeof v === 'boolean' branch so a formula with result: false returns ValueType.Boolean instead of throwing on false.text when falling through to the hyperlink check.

lib/xlsx/xform/sheet/cell-xform.js — parseClose

• Replace if (model.value) with if (model.value !== undefined) and add an else if (this.t === 'str') branch that sets model.result = '' for <c t="str"><f>...</f><v/></c>. Without this, a string-typed formula with empty-string result drops the result on read.
• Drive-by: extract the long shared-formula error message to a local so prettier doesn't restructure it across lines (avoids a pre-existing prettier vs. eslint comma-dangle conflict that fires whenever this file is edited).

Tests

spec/integration/issues/issue-2943-formula-falsy-result.spec.js — 9 assertions:

• value-to-value copy preserves result === 0, false, ''
• xlsx round-trip preserves result === 0, false, ''
• toCsvString returns '0' for result === 0 and 'false' for result === false
• toString returns '0' for result === 0

All pre-existing tests pass: 884 unit + 209 integration + 1 end-to-end.

Test plan

• npm run test:unit — 884 passing
• npm run test:integration — 209 passing (was 207; +2 from new spec)
• npm run test:end-to-end — 1 passing
• eslint clean on changed files
• prettier --check clean on changed files
• Pre-commit hook (lint-staged) succeeds without --no-verify