I have Burpless cucumbers and flowers on a few of them turned white instead of yellow.
White flowers are normal in some varieties of cucumber plants. The white color of the flowers on your Burpless cucumber plants could simply be from a lot of sun or genetic variation of the seeds or plant that you purchased and will probably not affect the cucumber shape, size or flavor. Make sure they're getting pollinated in the normal fashion and harvest the cucumbers frequently. This article may help you keep your cucumber plants healthy and happy: