fix(when): ensure then_enter_with accepts None

[mcous]

Fixes #135

[codecov]

Codecov Report

Merging #137 (78e789c) into main (c23d145) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #137   +/-   ##
=======================================
  Coverage   98.29%   98.30%           
=======================================
  Files          19       19           
  Lines         705      709    +4     
  Branches      120      121    +1     
=======================================
+ Hits          693      697    +4     
  Misses         10       10           
  Partials        2        2           

Impacted Files	Coverage Δ
decoy/call_handler.py	100.00% <100.00%> (ø)
decoy/stub_store.py	100.00% <100.00%> (ø)

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