Export custom fields without a value

export_custom_field_no_value.php

function export_option_givewp_field_api_fields() {
	?>
	<tr class="give-export-option-fields give-export-option-custom-field">
				<td scope="row" class="row-title">
					<label><?php esc_html_e( 'Custom Field Title:', 'give' ); ?></label>
				</td>
				<td class="give-field-wrap">
					<div class="give-clearfix">
						<ul class="give-export-option-custom-fields-ul">
							<!-- Custom field checkbox -->
							<li class="give-export-option-start">
								<label for="give-custom-field">
									<input type="checkbox" checked
									       name="give_give_donations_export_option[accept-mailing-list]"
									       id="accept-mailing-list"><?php _e( 'Accept Mailing List', 'give' ); ?>
								</label>
							</li>
						</ul>
					</div>
				</td>
			</tr>
	<?php
}

add_action( 'give_export_donation_fields', 'export_option_givewp_field_api_fields' );

/**
* Thi function sets the column header in the generated CSV file for the custom field. Add an if statement for each aditional field.
*/
function column_header_givewp_field_api_fields( $cols ) {
	if ( isset( $cols['accept-mailing-list'] ) ) {
		$cols['accept-mailing-list'] = __( 'Accept Mailing List', 'give' );
	}

	return $cols;

}

add_filter( 'give_export_donation_get_columns_name', 'column_header_givewp_field_api_fields' );

/**
* This function populates the CSV with the custom field data. Add an if statement for each aditional field.
*/
function export_givewp_field_api_fields( $data, $payment, $columns ) {
	if ( ! empty( $columns['accept-mailing-list'] ) ) {
		
		if ( metadata_exists( 'post', $payment->ID, 'accept_t_and_c' ) ) {
			$data['accept-mailing-list'] = 'Accepted';
		}
		
	}

	return $data;
}

add_filter( 'give_export_donation_data', 'export_givewp_field_api_fields', 10, 3 );