Calling wc_get_orders with a custom query parameter causes a DB error

[bswatson]

Describe the bug
An error is thrown when wc_get_orders is called with a custom query parameter which uses the woocommerce_order_data_store_cpt_get_orders_query filter to define additional meta_query` items.

To Reproduce
Steps to reproduce the behavior:

1. Create an order and assign meta data to it.

$order->update_meta_data( '_custom_meta_key', 'value' );

2. Create a filter to handle the incoming query var:

add_filter('woocommerce_order_data_store_cpt_get_orders_query', function ( $query, $query_vars ) {
	if ( ! empty( $query_vars['_custom_meta_key'] ) ) {
		$query['meta_query'][] = array(
			'key' => '_custom_meta_key',
			'value' => esc_attr( $query_vars['_custom_meta_key'] ),
		);
	}

	return $query;
}, 100, 2);

3. Execute wc_get_orders with the custom query var.

$orders = wc_get_orders( array( '_custom_meta_key' => 'value' ) );

4. Error is thrown and logged.

WordPress database error Unknown column 'wptests_postmeta.meta_key' in 'where clause' for query SELECT   wptests_posts.ID FROM wptests_posts  LEFT JOIN wptests_woocommerce_orders ON ( wptests_posts.ID = wptests_woocommerce_orders.order_id )  WHERE 1=1  AND ( 
  ( wptests_postmeta.meta_key = '_custom_meta_key' AND wptests_postmeta.meta_value = 'value' )
) AND wptests_posts.post_type IN ('shop_order', 'shop_order_refund') AND ((wptests_posts.post_status = 'wc-pending' OR wptests_posts.post_status = 'wc-processing' OR wptests_posts.post_status = 'wc-on-hold' OR wptests_posts.post_status = 'wc-completed' OR wptests_posts.post_status = 'wc-cancelled' OR wptests_posts.post_status = 'wc-refunded' OR wptests_posts.post_status = 'wc-failed')) GROUP BY wptests_posts.ID ORDER BY wptests_posts.post_date DESC LIMIT 0, 10

Expected behavior
$orders returns as an array with the expected order.

Versions

• WordPress version: 5.2.2
• WooCommerce version: 3.6.5
• Plugin version: develop @ 587ce29
• PHP version: 7.2

Additional context
Add any other context about the problem here.