Mysql doubt in fk null (Blank candidate election)

Question

I’m doing a voting system and I need you to take blank votes what is the best option to do this? agree to pass a null value in the cd_candidate column?

I can’t create a blank candidate in the candidate table

reason: they will have a lot of category and each category will have the white option the system picks the candidates of the category chosen by the voter

I could take a blank vote table and feed it in place of tb_votes but that would change everything in the count of votes by category.