fix string + string for PHP 8.X

[rrennick]

Changes proposed in this Pull Request:

This PR explicitly converts the string return values to float values.

Closes #38444.

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Update to PHP 8.1
2. Set all price tax settings to inclusive
3. Create an order with a product, shipping, and a coupon
4. Leave the order status as pending payment
5. Load the customer payment page
6. The customer payment page should show the correct amounts
7. No errors should be logged to the error log

[github-actions]

Test Results Summary

Commit SHA: 8c9ff70

Test 🧪	Passed ✅	Failed 🚨	Broken 🚧	Skipped ⏭️	Unknown ❔	Total 📊	Duration ⏱️
API Tests	259	0	0	2	0	261	0m 52s
E2E Tests	194	0	0	10	0	204	14m 47s

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To view the full API test report, click here.
To view the full E2E test report, click here.
To view all test reports, visit the WooCommerce Test Reports Dashboard.

[github-actions]

Hi @jorgeatorres,

Apart from reviewing the code changes, please make sure to review the testing instructions as well.

You can follow this guide to find out what good testing instructions should look like:
https://github.com/woocommerce/woocommerce/wiki/Writing-high-quality-testing-instructions

[jorgeatorres]

For testing, I used the following snippet as apparently string + string does work when string is numeric:

add_filter(
	'woocommerce_order_get_discount_tax',
	function( $val ) {
		return '';
	}
);

[jorgeatorres]

LGTM. Thank you @rrennick!