In 2.6, you could access the amount via $coupon->coupon_amount. Or legacy code incorrectly handles $coupon->amount instead. 7778583340/includes/class-wc-coupon.php (L102)
This PR handles both since the RCs and betas allowed `->amount` and I don't want to break anything that may be accessing it that way..
To Test:
* `phpunit --filter=test_coupon_backwards_compat_props_use_correct_getters`
`_deprecated_function` has conditional which is checking if third param (replacement) is null. Passing replacement as empty string to this function will fail this test and the end of triggered error will look like this: `Use instead.` instead of `with no alternative available.`. This PR removes all occurrences of empty strings in this function.
* Rename individual use, free shipping, and exclude sale items to match their meta names closer.
* Sanitize emails when using set_email_restrictions
* Remove some unnecessary explict true checks
* Move ID into _data with the rest of the fields
* Fixed some spacing issues
Claudio Sanches
Mike Jolley
Claudiu Lodromanean
Justin Shreve
Krzysztof Grabania
Aristeides Stathopoulos