VK provider returns error for users with empty 'email' field

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Hi
I've encountered an issue where `vk` provider will return "Cannot fetch user email" error for users which doesn't have this field filled in their VK profile.
I can't see why this is treated as an error and I think it should be fixed.

I can do a pull request fixing the issue but not sure what is the best way to implement it.
Fixing it directly in `vk` provider will probably break compatibility for some of existing goth users. I think some method on `vk` provider or separate provider should be created. Any opinions?

Steps to reproduce the issue:
1) register new VK account without specifying email. (I think it became possible since around 2018, but may be earlier).
2) Try to authorize him with `goth/provider/vk`

Expected:
`gothic.CompleteUserAuth(c.Writer, c.Request)` should return no error.

Actual:
`CompleteUserAuth` returns error `Cannot fetch user email`

Start by reading the goth/provider/vk implementation and tracing how gothic.CompleteUserAuth handles a VK user with no email. Reproduce the authorization flow with an empty email, then determine the compatibility-safe behavior for missing email and verify that CompleteUserAuth returns no error for that case.

Written by the indexing model from the issue text.