Handle empty and zero-weight density sample inputs

[BJMCox]

Fixes #555.

Return empty or all-zero sample vectors unchanged before weight normalization. Empty multiplicity conversion is already handled on main.

Main code: +3/-1.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.44%. Comparing base (6e8795f) to head (2a4fa0c).

Files with missing lines	Patch %	Lines
src/variates/density_sample.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #563      +/-   ##
==========================================
- Coverage   65.45%   65.44%   -0.01%     
==========================================
  Files         121      121              
  Lines        7060     7062       +2     
==========================================
+ Hits         4621     4622       +1     
- Misses       2439     2440       +1     

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