Null safety support

[lig]

This is a pretty naive migration to support null safety in redux.

In my opinion, it would be beneficial to have Store._state always be non-nullable. The issue is that the current logic in Store.teardown() looks hardly replaceable.

Notes:

• It's a common practice to create a separate branch in the repo for the null safety support. If done so, I'll retarget this PR accordingly.
• Until the coverage package is migrated to null safety this PR shouldn't be merged.

Todo:

• [ ] Retarget the PR to a new null-safety branch
• [ ] CI passes
• [x] Decide on the fate of the nullable state.
• [ ] Migrate coverage (not required).

[codecov-io]

Codecov Report

Merging #74 (5320710) into master (8872c0b) will decrease coverage by 1.86%. The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #74      +/-   ##
==========================================
- Coverage   92.10%   90.24%   -1.87%     
==========================================
  Files           2        2              
  Lines          38       41       +3     
==========================================
+ Hits           35       37       +2     
- Misses          3        4       +1     

Impacted Files	Coverage Δ
lib/src/store.dart	85.71% <85.71%> (-2.29%)	:arrow_down:

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