update default parameter of retrieve_password() for PHP8 compatibility

[afragen]

Quick PR to update retrieve_password() for PHP8 compatibility. The parameter default should be'' as line 3118 will throw a PHP Deprecation for passing null to trim().

Trac ticket: https://core.trac.wordpress.org/ticket/62298

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[github-actions]

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[afragen]

This issue occurs if the login button is clicked before data is entered. It happens to me when using some password managers if I try to click the remember me box before username.

[afragen]

Ping @peterwilsoncc 👋

[peterwilsoncc]

This looks good to me.

Thanks Andy!

[github-actions]

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Props afragen, peterwilsoncc.

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[SergeyBiryukov]

Thanks for the PR! Merged in r59312.