Fix NoMethodError when email domain is nil

[tnantoka]

Hi, and thanks for this great gem. I've been using it happily for years.

After the change in #42, some of my accounts started getting a NoMethodError
(undefined method 'casecmp?' for nil) on login. It turned out that some
Microsoft accounts (personal ones, in my case) don't return a mail, so
email_domain becomes nil and casecmp? raises before the other checks are
evaluated, even with skip_domain_verification set.

I fixed it with safe navigation, so a missing email simply does not match by
domain and falls through to the existing skip / JWT claim / error paths.

I saw #17 (comment) and I understand this gem isn't really aimed at personal accounts,
but since this change has no real downside (org accounts behave exactly the
same), I would be glad if you could consider merging it.

Thanks again!