Allow overriding % vaccinated when %vaccinated is not null

[beshaya]

Not sure why we had this limitation before?

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[codecov]

Codecov Report

Merging #1292 (aa29db0) into main (47fb105) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1292      +/-   ##
==========================================
+ Coverage   66.24%   66.26%   +0.02%     
==========================================
  Files         104      104              
  Lines        2829     2828       -1     
  Branches      514      513       -1     
==========================================
  Hits         1874     1874              
  Misses        804      804              
+ Partials      151      150       -1     

Impacted Files	Coverage Δ
.../calculator/prevalence/ManualPrevalenceDetails.tsx	66.66% <ø> (+2.66%)	⬆️

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