Add a single CometBFT dummy validator (backport #4116)

[mergify]

Describe your changes

Closes #4096

Only writes a dummy validator to CometBFT's genesis if the number of validators present is lower than 2.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

---

This is an automatic backport of pull request #4116 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 74.59%. Comparing base (65e1f67) to head (8931c89).
Report is 3 commits behind head on maint-0.46.

Files with missing lines	Patch %	Lines
crates/node/src/tendermint_node.rs	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           maint-0.46    #4118   +/-   ##
===========================================
  Coverage       74.58%   74.59%           
===========================================
  Files             341      341           
  Lines          107488   107492    +4     
===========================================
+ Hits            80170    80179    +9     
+ Misses          27318    27313    -5     

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