When `false` is set in `Configatron::Dynamic`, the call result becomes `nil`.

In the current latest version (v4.5.0):

```ruby
proc = Configatron::Proc.new { false }
proc.call
=> false

dynamic = Configatron::Dynamic.new { false }
dynamic.call
=> nil
```

When the value is false, the call result differs between Proc and Dynamic.
The call result should be the same.