Fix multicheck not saving when only the 0 option is checked - #1572

Description

CMB2_Sanitize::_is_empty_array() used array_filter() to check if a submitted array was empty. array_filter() also drops 0 and '0', so when the only checked multicheck option has the key 0, the value array( '0' ) is treated as empty. default_sanitization() then returns '' and CMB2_Field::save_field() deletes the value.

This PR switches the check to the existing CMB2_Utils::filter_empty() helper, which keeps 0 and '0' and still drops null, '', false and empty arrays.

Before:

$cleaned_up = array_filter( $to_check );

After:

// Keep 0 and '0'. They can be real values, like a multicheck option key.
$cleaned_up = CMB2_Utils::filter_empty( $to_check );

The check was added in 111bdad ("Fix empty arrays saving as a value") to stop arrays like array( '' ) from being saved. That still works the same after this change.

Motivation and Context

Fixes #1535.

Repro: a multicheck field with 'options' => array( 0 => 'Zero', 1 => 'One' ). Check only "Zero" and save. The page reloads with nothing checked and the meta is deleted. If any other option is also checked, it saves fine.

Risk Level

Low.

_is_empty_array() has one caller, CMB2_Sanitize::default_sanitization(), which every field type without its own sanitizer goes through. The only behavior change: a non-repeatable array value that holds only 0 or '0' is now saved instead of deleted. For that value, save_field() now calls update_data() and cmb2_save_field fires with $action = 'updated', where before it called remove_data() with $action = 'removed'. Nothing else changes:

• Empty arrays and arrays with only '', null or false are still treated as empty, so unchecking every option still removes the value.
• Arrays with any other value were already kept, so their output is the same.
• Post meta, the 'multiple' => true branch, group subfields, options pages and the REST API all use the same path, so they all get the fix.

Repeatable fields still drop '0' entries through a separate array_filter() in CMB2_Field::sanitization_cb(). That is older behavior and is not touched here.

Testing procedure

• Added Test_CMB2_Types::test_multicheck_sanitization_keeps_zero_value. It checks that array( '0' ) and array( 0 ) are kept, and that array() and array( '', null, false ) still return ''.
• Added Test_CMB2_Field::test_cmb2_multicheck_field_saves_zero_value. It goes through save_field(): saving only '0' stores array( '0' ) in post meta, saving '0' with another option stores both, and saving an empty array removes the meta.
• I checked that both tests fail with the fix reverted and pass with it.
• Full PHPUnit suite in wp-env (PHP 8.3, WordPress latest): 330 tests pass. composer phpcs is clean.
• Tested by hand in WordPress with the code from the issue, plus a multicheck_inline field, a multicheck inside a group and a multicheck on an options page. Checking only "Zero" now stays checked after saving, and unchecking everything clears the value.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.
• My code and pull requests meets the Contributing guidelines.

Screenshots

No UI changes.