fix: check outputs of miner transaction

[h4sh3d]

Fix #83

On coinbase transactions the amount is in clear with a RctType::Null, this PR fixes the logic to handle that case. Data for check_output_on_miner_tx test has been generated with monerod and monero-wallet-cli in regtest mode.

[codecov]

Codecov Report

Merging #96 (cef84cf) into main (8102ddc) will increase coverage by 0.5%.
The diff coverage is 95.6%.

@@           Coverage Diff           @@
##            main     #96     +/-   ##
=======================================
+ Coverage   75.8%   76.3%   +0.5%     
=======================================
  Files         16      16             
  Lines       2124    2169     +45     
=======================================
+ Hits        1610    1657     +47     
+ Misses       514     512      -2     

Impacted Files	Coverage Δ
src/blockdata/transaction.rs	58.6% <83.3%> (+0.2%)	⬆️
tests/blockdata.rs	99.0% <95.0%> (-1.0%)	⬇️
tests/recover_outputs.rs	100.0% <100.0%> (ø)
src/consensus/encode.rs	72.0% <0.0%> (+1.1%)	⬆️
src/util/key.rs	61.7% <0.0%> (+1.4%)	⬆️

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[TheCharlatan]

I'm surprised this was such a small change - nice change to test lines ratio :)

I don't think it is relevant, because the output will be unspendable, but what are the side effects, when a miner creates a 0 amount output, but the amount function returns None?

[h4sh3d]

I don't think it is relevant, because the output will be unspendable, but what are the side effects, when a miner creates a 0 amount output, but the amount function returns None?

Currently the reason I see to continue returning None in the library is because OwnedTxOut can be created with rct_sig_base set to null and the 0 amount is not a coinbase with 0.