fix stack panel GUI warnings

[carolhmj]

This is to cover a case that wasn't covered in #14452
Forum discussion: https://forum.babylonjs.com/t/control-is-using-height-in-percentage-mode-inside-a-vertical-stackpanel-warnings-after-update-to-6-25-0/45071/4

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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