If someone gets to the Unsubscribe page without an associated web session, and either
- the form doesn't make Email required
- the form doesn't contain a visible Email field
- the person has JS disabled (non-maliciously)
- the person purposely avoids JS validation
then you'll have a form post that doesn't contain an email address but still reaches Marketo.
On top of Sanford's comment, there is not much you can do, excepted to delete the lead from Marketo.
There's not much you can do about the existing blank email records, but there's plenty you can do to improve your email unsubscribe facility such as hiding the page from people who aren't "known" by using dynamic content.
Create a new segmentation, add a new segment "Known" and set the smart list rule as "Email address > is not empty". Use the segmentation on your landing page to control who can see the unsubscribe form.
I understand this may not be what you need, but point being there are plenty of ways to ensure your unsubscribe facility works how you want it to work.