Offer type text display doesn't handle custom offer types

commerce_discount_extra introduces the 'fixed_product_price' offer type, which sets a product to a specific price

When using this offer type, a PHP notice is thrown:

Error messageNotice: Undefined variable: offer_text in commerce_coupon_commerce_coupon_discount_value_display_alter() (line 1145 of commerce_coupon/commerce_coupon.module).

This is because, commerce_coupon doesn't account for instances in which $offer_text will never be set, because a product offer type other than fixed_amount or percentage is used.