Suggestion: `actions` property of a module is optional

[dallonf]

Some module's reducers only respond to global or higher-level actions, and don't have any actions of their own. It would be nice to not have to specify an actions: {} property in this case.

Probably want to leave selectors as required. On one hand, sometimes it's similarly useful to only expose a value within a higher-level selector - but this is an uncommon enough case that selectors: {} isn't annoying and highlights the unusual thing happening.