[BugFix] Fix stuck stats/metrics after requests are aborted

[njhill]

Ensure that we always return new stats from scheduler.

Includes small fix to block pool to exclude null block when calculating cache usage percentage.

Fixes #22545.

[github-actions]

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[gemini-code-assist]

Code Review

This pull request aims to fix an issue where scheduler statistics could get stuck if all requests in a step are aborted. The proposed change ensures that stats are always returned, even if there are no other engine outputs.

However, the implementation contains a critical bug related to operator precedence with the walrus operator (:=). In its current form, the code will either raise an AttributeError or cause a type error. I've provided a suggestion to fix this by adding the necessary parentheses to correctly capture the intended values before comparison.