Export EmptyValue symbol

[BrianHung]

Context

Exports EmptyValue.

How did you test your changes?

Types of changes

• Breaking change (a fix or a feature because of which an existing functionality doesn't work as expected anymore)
• New feature or improvement (a non-breaking change that adds functionality)
• Bug fix (a non-breaking change that fixes an issue)
• Additional language file, or a change to an existing language file (translations)
• Change to the documentation

Related issues:

1. Fixes Custom functions are unable to handle empty cell values. #1265
2. Reverts Hide EmptyValue from public APIs #323

Checklist:

• I have reviewed the guidelines about Contributing to HyperFormula and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement.
• My change is compliant with the OpenDocument standard.
• My change is compatible with Microsoft Excel.
• My change is compatible with Google Sheets.
• I described my changes in the CHANGELOG.md file.
• My changes require a documentation update.
• My changes require a migration guide.

[sequba]

Thank you for your contribution. Please, add a changelog entry, and we'll be ready to merge it.

[codecov]

Codecov Report

Merging #1296 (84750a7) into develop (d076d1e) will increase coverage by 0.00%.
Report is 1 commits behind head on develop.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1296   +/-   ##
========================================
  Coverage    97.20%   97.20%           
========================================
  Files          167      167           
  Lines        14295    14299    +4     
  Branches      3064     3064           
========================================
+ Hits         13895    13899    +4     
  Misses         395      395           
  Partials         5        5           

Files Changed	Coverage Δ
src/index.ts	100.00% <100.00%> (ø)
src/interpreter/index.ts	100.00% <100.00%> (ø)
src/interpreter/plugin/MatrixPlugin.ts	97.88% <100.00%> (ø)