Repair adaptive multi-proposal picking rules

[BJMCox]

Closes #534.

Handle zero-mass adaptive picking-rule updates without producing NaNs. Use uniform probabilities when replacing the active proposal weight leaves no mass.

The other adaptive multi-proposal tuner repairs from #534 are already on main.

Main code: +7/-2.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.42%. Comparing base (6e8795f) to head (bffb5a7).

Files with missing lines	Patch %	Lines
.../mcmc/mcmc_tuning/mcmc_adaptive_multiprop_tuner.jl	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
- Coverage   65.45%   65.42%   -0.03%     
==========================================
  Files         121      121              
  Lines        7060     7063       +3     
==========================================
  Hits         4621     4621              
- Misses       2439     2442       +3     

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