Page - update logic to apply sidebar-less layout

Describe the enhancement or change
Followup to https://github.com/patternfly/patternfly-react/pull/11760 where we apply logic to apply a modifier class for sidebar-less layout if sidebar={null} (tldr reason being to avoid breaking consumers who may rely on sidebar being explicitly undefined, but want to keep the layout as-is).

We want to update the logic in the next breaking change so that omitting the sidebar prop (or it otherwise being falsey) will apply this modifier class instead of having to explicitly pass null to the prop.

Is this request originating from a Red Hat product team? If so, which ones and is there any sort of deadline for this enhancement?

Any other information?

Review the sidebar-less layout logic in the Page component and the behavior introduced by pull request #11760. Trace how omitted, falsey, and explicitly null sidebar props are handled, then verify that the modifier class is applied for the intended cases without breaking existing consumers.