Fix: Avoid division by zero error on zero installments

[rafaelamiranda]

Description

This PR fixes a critical problem that causes a PHP DivisionByZeroError when the number of installments (_pagbank_credit_card_installments) is zero or not properly set.

When a payment fails or is rejected, this meta value is sometimes saved as zero, causing issues when the order details or emails are displayed, due to the division by the installment count.

Steps to reproduce:

• Create an order with PagBank payment and purposely fail the transaction or simulate a rejected payment.
• Try changing the order status from "Rejected" to other statuses directly from WooCommerce admin panel.
• A fatal error will occur because of the division by zero.

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