[Woo POS][Cash & Receipts] Validate empty cash user input as zero

[iamgabrielma]

Ref: pdfdoF-6fr#comment-7372-p2

Description

This PR updates the submit validation logic when processing cash, so if an empty textfield is submitted for validation, we assume that it's zero. This is done to handle 0-value carts with free products without the user having to specifically type "0" in the textfield.

Testing information

• Switch .acceptCashForPointOfSale on
• Process a POS transaction with a $0-cart value, select cash payment
• Tap the "Mark as Complete" button without typing anything in the textfield
• Observe that the transaction is processed normally

Screen.Recording.2025-01-23.at.08.25.37.mov

Receipts and the rest of the flow works as before, the only difference is we remove the need to type down 0 in the textfield.

---

• 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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[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 21.5. This milestone is due in less than 2 days. Please make sure to get it merged by then or assign it to a milestone with a later deadline.

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[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	pr14954-29b3990
	Version	21.4
	Bundle ID	com.automattic.alpha.woocommerce
	Commit	29b3990
	App Center Build	WooCommerce - Prototype Builds #12659

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[joshheald]

Works well, one small stylistic suggestion

[joshheald]

nit: I'd do this the other way around – it's easier to understand if the common case comes first (by checking .isNotEmpty,) then the fallback.

[iamgabrielma]

That makes sense, updated: 29b3990