Block in Strategy.configure can return nil

In situations like this:

```ruby
class MyStrategy
  include OmniAuth::Strategy
  configure do |c|
    c.baz = 123
    c.foo = ENV['BAR']
  end
end
```
the last assignment can return nil. This raises an error with unhelpful message `NoMethodError: undefined method 'each_pair' for nil:NilClass`