Fix ArgumentNullException thrown by BlockChain<T>.Fork() where it has incomplete states

[dahlia]

This fixes #454. I'm going to make another patch to remove the addressesToStrip parameter at all from IStore.ForkStateReferences() method (see also FIXME comment) to the master branch.

[codecov]

Codecov Report

Merging #466 into 0.5-maintenance will increase coverage by 7.71%.
The diff coverage is 100%.

@@                 Coverage Diff                 @@
##           0.5-maintenance     #466      +/-   ##
===================================================
+ Coverage            80.17%   87.88%   +7.71%     
===================================================
  Files                   61      195     +134     
  Lines                 1513    13686   +12173     
===================================================
+ Hits                  1213    12028   +10815     
- Misses                 271     1346    +1075     
- Partials                29      312     +283

Impacted Files	Coverage Δ
Libplanet.Tests/Blockchain/BlockChainTest.cs	98.41% <100%> (ø)
Libplanet/Blockchain/BlockChain.cs	94.49% <100%> (ø)
Libplanet/Net/Messages/BlockHashes.cs	100% <0%> (ø)
Libplanet/Net/NoSwarmContextException.cs	0% <0%> (ø)
Libplanet.Tests/Action/ActionEvaluationTest.cs	96.66% <0%> (ø)
Libplanet.Tests/Store/StoreTest.cs	98.17% <0%> (ø)
Libplanet/Blocks/Block.cs	86.38% <0%> (ø)
Libplanet/Net/PeerSetDelta.cs	85.36% <0%> (ø)
...planet/Blockchain/Policies/BlockPolicyExtension.cs	100% <0%> (ø)
Libplanet/Blocks/InvalidBlockTimestampException.cs	100% <0%> (ø)
... and 126 more