Remove all null initial states in redux reducers

[patcon]

Seems that to properly use prop-types, react doesn't play nicely with props that start as null but later become a specific form. This keeps us from using prop-types as effectively as we could be using them (e.g., making generous use of .isRequired and ensuring presence of defaultProps values whenever a value isn't required.)

You can trace the story from here, but there have been many workarounds over years:
facebook/react#2166 (comment)

My impression is that null isn't really supported in the ways we're using it in places like this:

https://github.com/pol-is/polisServer/blob/577dec6407470147b1c9043293ee30b85c6b335b/client-admin/src/reducers/mod_comments_accepted.js#L5-L8

where we then look for null to track when things are still loading:

https://github.com/pol-is/polisServer/blob/133e3d3b6c07ca1fddd27c414f25b382ffddc7d0/client-admin/src/components/conversation-admin/conversation-has-comments-check.js#L41

We should probably be doing one of two things:

1. something like this to track isLoading state outside looking for a null value:
   https://medium.com/stashaway-engineering/react-redux-tips-better-way-to-handle-loading-flags-in-your-reducers-afda42a804c6

2. Or using explicitNull custom prop-type from airbnb:
   Add isDefined and isNotNull parallels to isRequired facebook/prop-types#90 (comment)