Fix dataManager null errors

[aaronbeall]

Context

In some cases after updating a table with nestedRows enabled, HoT would throw a null error due to trying to iterate over missing data that it assumed was there.

How has this been tested?

Manually tested to verify HoT null errors no longer occur from these places in my app.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Additional language file or change to the existing one (translations)

Related issue(s):

Affected project(s):

• handsontable
• @handsontable/angular
• @handsontable/react
• @handsontable/vue
• @handsontable/vue3

Checklist:

• I have reviewed the guidelines about Contributing to Handsontable and I confirm that my code follows the code style of this project.
• I have signed the Contributor License Agreement
• My change requires a change to the documentation.

[adrianszymanski89]

Hi @aaronbeall

Thank you very much for your contribution. We will review your PR and update you when we check if we can merge it.

Before that, please add a changelog entry and tests for the fix. Thank you.