This closes #2449, correct blank cell comparisons

[EpicHigh]

Description

Blank cells were converted to numeric zero before every comparison. For a blank A1, this made A1="" return FALSE and A1="0" return TRUE.

Use an empty string when comparing a blank cell with text, FALSE when comparing it with a logical value, and zero otherwise. The change applies to all six comparison operators in either operand order.

Related Issue

Fixes #2449.

How Has This Been Tested

• The new regression test fails before the fix and passes after it.
• Tests cover text, numbers, logical values, cell references, IF, error handling, arithmetic, and text joining.
• go test ./... -count=1 -timeout 20m
• go test -race ./... -run '^TestCalc' -count=1 -timeout 10m
• go vet ./...

Tested with Go 1.27.1 on macOS arm64.

A full race-enabled run was also attempted, but the process was killed before completion.

Types of changes

• Bug fix

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.72%. Comparing base (da3f2df) to head (3ebaf3a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2457   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          32       32           
  Lines       32838    32845    +7     
=======================================
+ Hits        32748    32755    +7     
  Misses         88       88           
  Partials        2        2           

Flag	Coverage Δ
unittests	99.72% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.