RFR: 8318019: GenShen: Fix assertion to allow empty evacuation cycles

[William Kemper]

On Wed, 11 Oct 2023 16:27:18 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> Assertion needs to allow for gc phase EVACUATION even if no forwarding objects because we are only promoting in place.

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/337#pullrequestreview-1674731832