Fix grid view crash when task groups change across DAG runs

[jaymasiwal]

What does this PR do?

Fixes a crash in the Grid View UI when task group structure changes across DAG runs.
Previously, empty task groups could be yielded without any task instance details,
leading to inconsistent aggregation and a UI failure.

Why is this needed?

When a task group exists in one DAG run but has no task instances in another,
the grid aggregation logic attempted to render an empty group, which caused
the grid view to break.

How was this fixed?

The task group node is now yielded only when it has at least one child with
task instance details, ensuring aggregation data is always consistent.

Related issue

Fixes #61208

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