fix(metadata): actually use None for unset height

[jansegre]

Acceptance criteria

• use None instead of 0 in BaseTransaction.get_metadata when creating a new TransactionMetadata
• add validations to save/get on TransactionStorage so we make sure blocks always have a height when being saved

[codecov]

Codecov Report

Merging #671 (ae441ba) into master (441c255) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   83.84%   83.91%   +0.06%     
==========================================
  Files         246      246              
  Lines       20340    20365      +25     
  Branches     2746     2749       +3     
==========================================
+ Hits        17055    17090      +35     
+ Misses       2712     2705       -7     
+ Partials      573      570       -3     

Impacted Files	Coverage Δ
hathor/transaction/base_transaction.py	90.62% <100.00%> (+0.01%)	⬆️
hathor/transaction/storage/transaction_storage.py	91.04% <100.00%> (-0.02%)	⬇️

... and 11 files with indirect coverage changes