The method `LoadBalancer#connection_error?` would unwrap errors that
inherit from ActiveRecord::StatementInvalid and
ActionView::Template::Error, in an attempt to see if any connection
errors were wrapped. When given an error without a cause, such as
ActiveRecord::RecordNotUnique, it would fail, as `cause` on the error
returns `nil`. This in turn would manifest in various tests, such as
those for the `ApplicationRecord` model.

This commit fixes these issues by just returning `false` when there is
no error cause.

Changelog: fixed