Accept remote's closing fee when we have nothing at stake

[pm47]

If we have nothing at stake there is no need to negotiate and we accept
their fee right away.

⚠️ This is a small change but a bug here would be costly.

[codecov-io]

Codecov Report

Merging #1633 (bfdca8d) into master (0e5ec4d) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1633      +/-   ##
==========================================
+ Coverage   85.88%   85.94%   +0.06%     
==========================================
  Files         151      151              
  Lines       11417    11456      +39     
  Branches      492      494       +2     
==========================================
+ Hits         9805     9846      +41     
+ Misses       1612     1610       -2     

Impacted Files	Coverage Δ
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.06% <100.00%> (+0.01%)	⬆️
...main/scala/fr/acinq/eclair/router/Validation.scala	91.41% <0.00%> (-1.50%)	⬇️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	92.85% <0.00%> (-1.06%)	⬇️
...nq/eclair/blockchain/electrum/ElectrumWallet.scala	80.25% <0.00%> (-0.26%)	⬇️
.../scala/fr/acinq/eclair/blockchain/Monitoring.scala	100.00% <0.00%> (ø)
.../scala/fr/acinq/eclair/payment/PaymentPacket.scala	91.42% <0.00%> (ø)
...c/main/scala/fr/acinq/eclair/channel/Helpers.scala	96.23% <0.00%> (+0.01%)	⬆️
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	92.00% <0.00%> (+0.13%)	⬆️
.../acinq/eclair/blockchain/bitcoind/ZmqWatcher.scala	99.27% <0.00%> (+0.15%)	⬆️
... and 3 more