[Woo POS] Update connection status when reader turned off

[joshheald]

Closes: #14992
Merge after: #14990

Description

When we get a reader status change from the underlying CardPresentPaymentStore, we should always republish that to POS.

Previously, we would treat empty arrays of readers from the store (nil in the connectedReaderPublisher) as something we don’t have to pass on, when in reality they likely represent a disconnection originating outside the app (e.g. turning the reader off, bluetooth failure, or the battery running out.)

This PR updates the handling to treat these cases as a disconnected reader, so now the UI will update accordingly.

Steps to reproduce

1. Launch the app and open POS
2. Connect your card reader
3. Turn the reader off
4. Observe that the status updates to show Connect your reader again.

Testing information

This makes other bugs reproducible: #13738 and #13876 (which are probably the same issue) – I'll tackle these separately.

I attempted to add unit tests for this, but the nature of the facade layer made this unfeasible without major changes. It's a problem with the combination of async and combine...

Screenshots

update.card.reader.status.when.turned.off.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.

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• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.

[wpmobilebot]

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

	App Name	WooCommerce iOS
	Build Number	pr14994-e525195
	Version	21.5
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	e525195
	App Center Build	WooCommerce - Prototype Builds #12714

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[iamgabrielma]

LGTM 🚢

Tested on iPad Pro 12.9 w/ iOS 17.6.1