Fix X.509 certificate creation with blank email addresses

What does this MR do and why?

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1832009

ActiveRecord::RecordInvalid: Validation failed: Email can't be blank, Email is invalid (ActiveRecord::RecordInvalid)
  from active_record/validations.rb:84:in `raise_validation_error'
  from active_model/validations.rb:412:in `validate!'
  from app/models/application_record.rb:71:in `block in safe_find_or_create_by!'
  from <internal:kernel>:90:in `tap'
  from app/models/application_record.rb:68:in `safe_find_or_create_by!'
  from active_record/relation/delegation.rb:122:in `public_send'
  from active_record/relation/delegation.rb:122:in `block in method_missing'
  from active_record/relation.rb:939:in `_scoping'
  from active_record/relation.rb:467:in `scoping'
  from active_record/relation/delegation.rb:122:in `method_missing'
  from app/models/x509_certificate.rb:33:in `safe_create!'
  from lib/gitlab/x509/signature.rb:27:in `x509_certificate'
  from lib/gitlab/x509/commit.rb:20:in `attributes'

Problem

X.509 signature verification fails with ActiveRecord::RecordInvalid when certificates contain blank email addresses in subjectAltName.

Solution

Add email validation to return nil when certificate_email is blank.

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