fix: allow empty public keys

[cmwaters]

Description

Empty public keys would throw an error when calling SetSignatures. Public keys are optional and only needed in the first transaction to set it in state.

This should not break consensus and thus is okay as a minor release

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Author Checklist

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I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
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• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

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your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[codecov-commenter]

Codecov Report

Attention: 52 lines in your changes are missing coverage. Please review.

Comparison is base (e04d485) 65.55% compared to head (fdfb76a) 65.54%.
Report is 4 commits behind head on release/v0.46.x-celestia.

Additional details and impacted files

@@                     Coverage Diff                      @@
##           release/v0.46.x-celestia     #371      +/-   ##
============================================================
- Coverage                     65.55%   65.54%   -0.01%     
============================================================
  Files                           666      666              
  Lines                         71180    71213      +33     
============================================================
+ Hits                          46661    46680      +19     
- Misses                        21936    21945       +9     
- Partials                       2583     2588       +5     

Files	Coverage Δ
x/upgrade/keeper/keeper.go	83.72% <100.00%> (ø)
x/auth/tx/builder.go	82.35% <70.00%> (+0.29%)	⬆️
x/params/types/consensus_params.go	0.00% <0.00%> (ø)
x/auth/client/cli/tx_multisign.go	0.00% <0.00%> (ø)
baseapp/abci.go	59.93% <11.11%> (-0.67%)	⬇️
baseapp/options.go	56.66% <0.00%> (-2.99%)	⬇️
baseapp/params.go	80.00% <0.00%> (-20.00%)	⬇️
baseapp/baseapp.go	73.43% <5.55%> (-3.03%)	⬇️

... and 1 file with indirect coverage changes