fix(grid): address null TypeError on focusing a grid with no data and column headers - master

[ddaribo]

Closes #16517

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[Copilot]

Pull request overview

This PR fixes a null TypeError that occurs when focusing a grid with no data and column headers. The issue arises when grid.navigation.activeNode is null, causing attempts to access its row property to fail.

Key Changes:

• Added optional chaining (?.) to safely access activeNode.row in two header components
• Added a test case to verify the fix prevents the TypeError

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
grid-header-row.component.ts	Added optional chaining to prevent null reference error when accessing activeNode.row in isLeafHeaderAriaHidden getter
grid-header-group.component.ts	Added optional chaining to prevent null reference error when accessing activeNode.row in ariaHidden getter
column-group.spec.ts	Added test case to verify no errors are thrown when focusing a grid with no active node (contains a critical bug in the test implementation)

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