Shipping Label: Fix incorrect height for predefined packages when unavailable

[itsmeichigo]

Closes WOOMOB-682

Description

This PR fixes the incorrect package dimensions when height is unavailable.

Previously we were using the last dimension as the height. This is incorrect when the API response for packages doesn't include a height for a package.

The solution is to access dimensions by safe indices and return nil if unavailable.

Another problem is when sending a 0 height, the label rates request would fail with an unexpected value error. This PR adds a workaround to set a default value of 0.25 in this case, the rates returned would still be the same for predefined packages [p1750923325060079-slack-C05VBLKHHV1].

Testing steps

1. Log in to a store with Woo Shipping extension set up.
2. Navigate to the Orders tab and select a paid order with physical products.
3. Select Create shipping label and tap Add a package.
4. Switch to the Carrier tab > UPS and confirm that the UPS letter package doesn't display a height.
5. Select UPS letter package and confirm that the corresponding rates are loaded successfully.
6. Proceed to purchase the label and confirm that purchase completes successfully.

Testing information

Tested and confirmed with simulator iPhone 16 iOS 18.4.

Screenshots

Simulator.Screen.Recording.-.iPhone.16.-.2025-06-26.at.15.09.04.mp4

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• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 22.6 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce iOS Prototype by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce iOS Prototype
	Build Number	30755
	Version	PR #15812
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	ad88973
	Installation URL	0ao7hahp2lego

Automatticians: You can use our internal self-serve MC tool to give yourself access to those builds if needed.

[RafaelKayumov]

LGTM. Works as described.