Fix zero-mass quantiles and low-ESS bridge uncertainty

[BJMCox]

Repairs two weighted-sample failures found while auditing the code merged through #564.

• Ignore zero-mass NaNs when computing empirical quantiles.
• Draw at least two independent bridge proposal samples so the corrected variance has a defined denominator.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.43%. Comparing base (5ba992e) to head (9886f19).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #577   +/-   ##
=======================================
  Coverage   65.43%   65.43%           
=======================================
  Files         121      121           
  Lines        7060     7060           
=======================================
  Hits         4620     4620           
  Misses       2440     2440           

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