Prevents JavaScript errors by checking zxcvbn is defined before calling.
Changes `wp.passwordStrength.meter()` to return `-1` if the strength of the password is unknown.
On the user profile screen, `generatePassword()` checks if the user has entered the password before setting the value of the password input box.
Props peterwilsoncc, adamsilverstein.
Fixes#34905.
Built from https://develop.svn.wordpress.org/trunk@37940
git-svn-id: http://core.svn.wordpress.org/trunk@37881 1a063a9b-81f0-0310-95a4-ce76da25c4cd