如何将空字符串设置为空值以允许在可选字段中重复空字符串？

I have an email field that's optional. The regex works fine accepting the empty string (^$) but now the issue is that the empty string is considered a unique entry. It will only allow one user to register without entering an email address. I know I have to set it to null, but not sure how.I have an email field that's optional. The rege