Don't convert zero values to empty string in woocommerce_process_shop_order_meta()

PHP's `empty()` function returns true for 0, 0.00, "0.00" and other 0 variants. 

As a result, `woocommerce_process_shop_order_meta()` will convert item meta with a `0.00` or similar value to an empty string, which can break things that expect a float or number. 

These commits fix that.