[staking] make contract staking indexer nullable

[envestcc]

Description

To improve flexibility and testability, make the contract staking indexer param in staking.NewProtocol() nullable.

Fixes #(issue)

Type of change

Please delete options that are not relevant.

• [] Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• Code refactor or improvement
• [] Breaking change (fix or feature that would cause a new or changed behavior of existing functionality)
• [] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• make test
• [] fullsync
• [] Other test (please specify)

Test Configuration:

• Firmware version:
• Hardware:
• Toolchain:
• SDK:

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Merging #3883 (7037b26) into master (e1f0636) will increase coverage by 0.23%.
The diff coverage is 67.58%.

❗ Current head 7037b26 differs from pull request most recent head b498078. Consider uploading reports for the commit b498078 to get more accurate results

@@            Coverage Diff             @@
##           master    #3883      +/-   ##
==========================================
+ Coverage   75.38%   75.61%   +0.23%     
==========================================
  Files         303      326      +23     
  Lines       25923    27802    +1879     
==========================================
+ Hits        19541    21023    +1482     
- Misses       5360     5694     +334     
- Partials     1022     1085      +63     

Impacted Files	Coverage Δ
action/protocol/execution/evm/evm.go	43.52% <0.00%> (-2.95%)	⬇️
action/protocol/execution/evm/evmstatedbadapter.go	66.77% <ø> (ø)
action/protocol/poll/consortium.go	0.00% <0.00%> (ø)
action/protocol/poll/staking_committee.go	43.85% <0.00%> (ø)
...tion/protocol/staking/contractstake_bucket_type.go	0.00% <0.00%> (ø)
api/grpcserver.go	86.40% <0.00%> (ø)
api/loglistener.go	25.00% <0.00%> (ø)
api/web3server_marshal.go	93.21% <ø> (ø)
api/websocket.go	5.17% <0.00%> (ø)
blockchain/config.go	74.54% <ø> (ø)
... and 49 more

... and 4 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
17.7% Duplication

Catch issues before they fail your Quality Gate with our IDE extension SonarLint