Consider null as a valid type
Consider null as a valid type

I recently run into [an issue](https://github.com/zalmoxisus/redux-devtools-extension/issues/114#issuecomment-218821024) when returning an action type as `null`, however I don't know the reason of [this change](https://github.com/gaearon/redux-devtools-log-monitor/commit/b132439121dc63c9681ff2d148999804690c7217) so maybe @anibali wants to have a look to this PR as well :)
gaearon/redux-devtools-log-monitor