dnsdist: Display DynBlocks' default action, None, as the global one

[rgacogne]

Short description

The default action for a dynamic block is Action::None, which doesn't mean that nothing will be done but that the global action will be applied instead. We failed to display that correctly in the console and via the API.
Reported by ph1 (thanks!).

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[phonedph1]

Nice! Thanks. Tested and works as expected now.