Fix empty backup code when appending codes for a user with none

[faithcoder]

What?

Generating backup codes with 'method' => 'append' for a user who has no existing codes no longer stores an extra, empty code that can never be validated or removed.

Fixes #956

Why?

generate_codes() cast get_user_meta( ..., true ) directly to an array. For a user with no stored codes that meta value is '', and (array) '' evaluates to array( '' ) rather than an empty array, so an empty string was permanently appended ahead of the real hashes. codes_remaining_for_user() then reports one more code than the user actually holds, causing two problems:

• A user with zero usable codes is still offered recovery codes at login.
• The low-codes warning reports one code too many.

How?

Replaced the bare (array) cast with an is_array() guard before assigning the existing hashes, matching the pattern already used by codes_remaining_for_user() and validate_code(). Added a regression test for the append-with-no-existing-codes case.

Use of AI Tools

AI assistance: Yes
Tool(s): OpenCode
Model(s): deepseek-v4-pro
Used for: Root-cause analysis, fix implementation, and test authoring; the final diff and validation were reviewed by the contributor.

Testing Instructions

1. npm run composer -- test -- --filter Tests_Two_Factor_Backup_Codes
2. npm test (full suite)
3. Manual reproduction from the issue:

$user  = get_user_by( 'login', 'someone' ); // no backup codes yet
$codes = Two_Factor_Backup_Codes::get_instance()->generate_codes( $user, array( 'method' => 'append' ) );

count( $codes );                                              // 10
count( get_user_meta( $user->ID, '_two_factor_backup_codes', true ) ); // 10 (was 11)
Two_Factor_Backup_Codes::codes_remaining_for_user( $user );   // 10 (was 11)

Screenshots or screencast

Not applicable (no UI changes).

Changelog Entry

Fixed - Generating backup codes with the append method no longer stores an empty entry for users with no existing codes.

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Co-authored-by: faithcoder <faithcoder@git.wordpress.org>
Co-authored-by: dknauss <dpknauss@git.wordpress.org>

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