Issue number: resolves #30081

What is the current behavior?

In handlePageTransition, when a page component is missing the required <ion-page> wrapper, enteringEl (enteringViewItem.ionPageElement) is undefined. The code correctly detects this and logs a warning, but does not return early. Execution continues to isViewVisible(enteringEl), which calls enteringEl.classList.contains(...) on undefined, causing an unhandled TypeError.

This crash prevents the router outlet from completing the transition and can leave it in a broken state where subsequent navigations no longer work.

What is the new behavior?

• Add return after the console.warn in handlePageTransition when enteringEl is undefined, so the function exits gracefully instead of crashing.
• Update isViewVisible to accept HTMLElement | undefined and return false for undefined, as a defensive measure.
• Add unit tests to verify that handlePageTransition warns and returns early when enteringEl is undefined, and does not call commit (transition).

Does this introduce a breaking change?

• Yes
• No

Other information

The existing console.warn at this location indicates the team already anticipated this scenario — a page without <ion-page> — but the missing return means the warning is immediately followed by a crash. This fix simply completes the intended error handling.

Without the fix, isViewVisible(undefined) throws TypeError: Cannot read properties of undefined (reading 'classList'). This unhandled error breaks the router outlet's internal state, causing all subsequent page transitions to silently fail. The user sees a frozen UI with no error feedback, which is significantly worse than the degraded-but-functional behavior the warning was designed to allow.