Allow specifying a bitcoin wallet with an empty name

[pm47]

We make the distinction between not defining a bitcoin wallet name, and using an bitcoin wallet with an empty name.

This remains mostly backward compatible, as:

• by default eclair.bitcoind.wallet is undefined so we keep using the default loaded wallet as before
• if user already set eclair.bitcoind.wallet, then eclair will use that wallet.
• only in the case where the user left eclair.bitcoind.wallet undefined, and if bitcoin had only one wallet with a non-empty name, then eclair will fail right at startup with an explicit error

[codecov-commenter]

Codecov Report

Merging #2737 (91b3286) into master (8d42052) will increase coverage by 0.06%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #2737      +/-   ##
==========================================
+ Coverage   85.85%   85.92%   +0.06%     
==========================================
  Files         215      215              
  Lines       17823    17824       +1     
  Branches      761      796      +35     
==========================================
+ Hits        15302    15315      +13     
+ Misses       2521     2509      -12     

Files Changed	Coverage Δ
...ir-core/src/main/scala/fr/acinq/eclair/Setup.scala	74.85% <ø> (ø)
.../scala/fr/acinq/eclair/blockchain/Monitoring.scala	100.00% <100.00%> (ø)
...chain/bitcoind/rpc/BasicBitcoinJsonRPCClient.scala	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes