Prevent double-processing new option values/settings

[Roy-Orbison]

A possible solution to the gotcha of relying on the behaviour of update_option(), which register_setting() does:

If the option does not exist, it will be created.

It is not obvious to developers that their option will, currently, be sanitised twice in that case. This can easily lead to data corruption, plugin crashes, etc.

This patch is a little bit of a hack inasmuch as any other underscore-prefixed private function, but I don't see another DRY way to bypass only that processing which has already been done.

Trac ticket: https://core.trac.wordpress.org/ticket/21989

---

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[Roy-Orbison]

Can also confirm that new unit test fails on parent commit of this PR:

$ vendor/bin/phpunit --verbose tests/phpunit/tests/option/updateOption.php 
Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml
Not running ajax tests. To execute these, use --group ajax.
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.
PHPUnit 9.6.16 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.2.10-2ubuntu1
Configuration: /home/roy/path/to/wordpress-develop/phpunit.xml.dist
Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

.........F                                                        10 / 10 (100%)

Time: 00:00.051, Memory: 42.50 MB

There was 1 failure:

1) Tests_Option_UpdateOption::test_stored_sanitized_value_from_update_of_nonexistent_option_should_be_same_as_that_from_add_option
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'x_y'
+'x_y_y'

/home/roy/path/to/wordpress-develop/tests/phpunit/tests/option/updateOption.php:249