feat: fix GetBlock for null rounds by returning nil

[Vladmair]

关联的Issues (Related Issues)

close

改动 (Proposed Changes)

附注 (Additional Info)

自查清单 (Checklist)

在你认为本 PR 满足被审阅的标准之前，需要确保 / Before you mark the PR ready for review, please make sure that:

• 符合Venus项目管理规范中关于PR的相关标准 / The PR follows the PR standards set out in the Venus project management guidelines
• 具有清晰明确的commit message / All commits have a clear commit message.
• 包含相关的的测试用例或者不需要新增测试用例 / This PR has tests for new functionality or change in behaviour or not need to add new tests.
• 存在兼容性问题（接口, 配置，数据，灰度），如果存在需要进行文档说明 / This PR has compatibility issues (API, Configuration, Data, GrayRelease), if so, need to be documented.
• 包含相关的的指南以及文档或者不需要新增文档 / This PR has updated usage guidelines and documentation or not need
• 通过必要的检查项 / All checks are green

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 13 lines in your changes missing coverage. Please review.

Project coverage is 23%. Comparing base (a550fc3) to head (651409a).
Report is 5 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #6415   +/-   ##
======================================
  Coverage      23%     23%           
======================================
  Files         740     740           
  Lines       76724   76729    +5     
======================================
+ Hits        17930   17946   +16     
+ Misses      55944   55938    -6     
+ Partials     2850    2845    -5     

[simlecode]

lgtm