Optimize shopping cart page to avoid flash of unavailable sizes(#40455)

[Madhavendrasinh-Vaghela]

Fixes #40455

Description
This PR fixes a UI issue in the e-commerce example where all size options would momentarily flash as "unavailable" (crossed out) when switching between shirt colors.

Motivation
When a user selects a different color, the application fetches the size data for that color asynchronously. During this network request, the sizes object is undefined. The previous logic !shirts[selected.id].sizes['SIZE'] evaluated to true during this loading and undefined state, causing the UI to incorrectly mark all sizes as unavailable.

Changes
Updated the [class] binding logic for all size options in
examples/bind/ecommerce.amp.html.

• Old Logic: Checked only for the absence of the specific size.
• New Logic: Adds a guard check shirts[selected.id].sizes && ... to ensure the sizes data exists before determining if a specific size is unavailable.

Verification
1.Open
examples/bind/ecommerce.amp.html .

2. Select a different color (e.g., "Brown").

3. Observe that size options remain in their neutral state during the fetch and update correctly once data is loaded, without flashing the "unavailable" style.

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