fix(ui5-dynamic-page): ignore zero-box focus sentinels in scroll handler

[NakataCode]

Problem

On Shift+Tab from a growing button, the Table's roving-tabindex moves focus through a transient zero-height sentinel element before landing on the real row.

DynamicPage.onContentFocusIn captured that sentinel and called scrollIntoView on it, scrolling the container down and hiding the focused row.

Solution

Guard the handler to bail early when the focus target has no layout box on either axis:

if (target.offsetWidth === 0 || target.offsetHeight === 0) return;

Real focusables always have a non-zero box on both axes; sentinels do not.

Fixes: #13935

[sap-ui5-webcomponents-release]

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