Edit only if the field is blank

edit_only_if_blank.php

add_filter( 'user_meta_field_config', 'editOnlyBlank', 10, 2 );
function editOnlyBlank( $field, $fieldID ) {
    global $userMeta;
    
    // Change Target_Field_ID
    if ( $fieldID <> 'Target_Field_ID' ) return $field;

    // Uncomments this line if an admin are allowed to edit value
    // if ( $userMeta->isAdmin() ) return $field;
        
    $field['read_only'] = ! empty( $field['field_value'] ) ? true : false;
    
    return $field;
}