fix: allow transactions from the "null" account in tenure change blocks

[brice-stacks]

Thsi will fix several integration tests that have been failing.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.87%. Comparing base (cda6a5e) to head (62b5727).
⚠️ Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
stacks-node/src/tests/nakamoto_integrations.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #6713      +/-   ##
===========================================
- Coverage    80.03%   79.87%   -0.16%     
===========================================
  Files          577      577              
  Lines       357651   357652       +1     
===========================================
- Hits        286235   285685     -550     
- Misses       71416    71967     +551     

Files with missing lines	Coverage Δ
stacks-node/src/tests/nakamoto_integrations.rs	41.16% <66.66%> (+8.25%)	⬆️

... and 90 files with indirect coverage changes

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