Allow parameter contexts for uninitialized parameters

[thangleiter]

Currently, using the set_to context fails on exit for parameters whose value does not pass the set parser or validator. The only -- but common -- case where this happens are uninitialized parameters, in which case the value is None.

This change catches exceptions when setting the original value and then sets the cache value instead. The exception catch is very broad, but as far as I can tell, validators can raise any kind of exception they like. Only doing this if original_value is None might be a bit safer -- are there other cases?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.62%. Comparing base (e606790) to head (c6f543a).
Report is 46 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6074      +/-   ##
==========================================
+ Coverage   67.55%   67.62%   +0.07%     
==========================================
  Files         351      351              
  Lines       30492    30560      +68     
==========================================
+ Hits        20598    20667      +69     
+ Misses       9894     9893       -1     

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