wp-settings- and wp-settings-time- cookies set on every admin request if user settings empty

[kkmuffme]

if the WP user settings are empty, WordPress will set the wp-settings-time- on every admin request again and again, and also tries to delete the wp-settings- cookie, since PHP treats setcookie with empty value as expired cookie, ignoring the provided expiration.
This behavior was different in PHP<7 and still incorrectly documented ​https://github.com/php/doc-en/issues/4614

Other places that use that cookie already have this correctly implemented though:
​https://github.com/WordPress/wordpress-develop/blame/trunk/src/wp-includes/option.php#L1917
​https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/pluggable.php#L1145

Just setting the value to a space, like it's done in the other places, ensures it will not try to delete/set the cookie(s) on every request, but validation still works, since spaces are removed before comparison with option value.

…on every request, if settings are empty

Trac ticket: https://core.trac.wordpress.org/ticket/63280