make deal state channel id nilable

[dirkmc]

Addresses filecoin-project/go-data-transfer#143

TODO:

• ClientDealState
• ProviderDealState

[codecov-io]

Codecov Report

Merging #490 (dc2b0bc) into master (e50bca4) will increase coverage by 0.17%.
The diff coverage is 88.41%.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   65.16%   65.32%   +0.17%     
==========================================
  Files          50       51       +1     
  Lines        3266     3408     +142     
==========================================
+ Hits         2128     2226      +98     
- Misses        912      954      +42     
- Partials      226      228       +2     

Impacted Files	Coverage Δ
...etrievalmarket/impl/providerstates/provider_fsm.go	6.46% <0.00%> (ø)
retrievalmarket/types.go	41.47% <ø> (ø)
...rievalmarket/impl/requestvalidation/revalidator.go	76.23% <22.23%> (-3.33%)	⬇️
retrievalmarket/impl/client.go	68.40% <100.00%> (ø)
retrievalmarket/impl/clientstates/client_fsm.go	74.34% <100.00%> (ø)
retrievalmarket/impl/clientstates/client_states.go	92.41% <100.00%> (+0.10%)	⬆️
...ievalmarket/impl/providerstates/provider_states.go	90.70% <100.00%> (+0.46%)	⬆️
retrievalmarket/migrations/migrations.go	71.43% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e50bca4...dc2b0bc. Read the comment docs.

[nonsense]

lgtm