Fixes #59663

SliverGridRegularTileLayout.computeMaxScrollOffset did not account for a child count of 0.
This would either produce a crash or report the wrong value.

SliverGrid.builder(
  itemCount: 0,
  itemBuilder: (_, __) => Container(),
  gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
    crossAxisCount: 1,
    mainAxisSpacing: 10,
    childAspectRatio: 2.1,
  ),
),

This case would cause a crash from a negative scroll extent. 💀

SliverGrid.builder(
  itemCount: 0,
  itemBuilder: (_, __) => Container(),
  gridDelegate: const SliverGridDelegateWithMaxCrossAxisExtent(
    maxCrossAxisExtent: 30,
  ),
),

This would just report the wrong value > 0. 🙃

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• I listed at least one issue that this PR fixes in the description above.
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• I added new tests to check the change I am making, or this PR is test-exempt.
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