Channel queries: Encode empty arrays as UNCOMPRESSED

[sstone]

c-lightning fails to decode empty arrays of scids or timestamps with an encoding type set to COMPRESSED_ZLIB.
The spec is not specific enough on whether this is valid or not, so we'll set the encoding type of empty arrays to UNCOMPRESSED.

This is PR supersedes #1280

[codecov-io]

Codecov Report

Merging #1292 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1292      +/-   ##
==========================================
+ Coverage   77.18%   77.33%   +0.15%     
==========================================
  Files         143      143              
  Lines        9976     9981       +5     
  Branches      408      418      +10     
==========================================
+ Hits         7700     7719      +19     
+ Misses       2276     2262      -14

Impacted Files	Coverage Δ
...src/main/scala/fr/acinq/eclair/router/Router.scala	93.54% <100%> (+1.25%)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	83.65% <0%> (+0.09%)	⬆️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	90.68% <0%> (+0.35%)	⬆️
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	52.13% <0%> (+0.85%)	⬆️
...clair/blockchain/electrum/ElectrumClientPool.scala	78.49% <0%> (+4.3%)	⬆️

[t-bast]

LGTM, sounds like a reasonable alternative until we hear back from the other implementations.

[sstone]

@cdecker this fixes an interop issue between eclair and c-lightning (see ElementsProject/lightning#3425 and lightning/bolts#729)

[cdecker]

Great, thanks for following up on this @sstone and @t-bast 👍