Bugfix in get_viscosity_kinematic

[rkingsbury]

Summary

A recent change to get_salt_dict (see #258 ) created a problem with get_viscosity_kinetmatic that was not previously being tested. Before #258 , get_salt() would always return a salt object, even for pure water (in which case the anion and cation were OH- and H+. After the PR, get_salt returns None. This PR handles this possibility and adds tests to ensure get_viscosity_kinematic works correctly even for pure water.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 84.72%. Comparing base (b21d273) to head (256bc5d).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pyEQL/solution.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #281      +/-   ##
==========================================
+ Coverage   83.89%   84.72%   +0.83%     
==========================================
  Files           9        9              
  Lines        1459     1460       +1     
  Branches      251      251              
==========================================
+ Hits         1224     1237      +13     
+ Misses        206      193      -13     
- Partials       29       30       +1     

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