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?

检查 Page 组件中无侧边栏布局的逻辑，以及 pull request #11760 引入的行为。跟踪省略的、falsey 的和显式设置为 null 的 sidebar props 是如何处理的，然后验证 modifier class 是否会在预期情况下应用，同时不破坏现有的 consumer。