wrap <slot> updates in conditional

[Rich-Harris]

Well, I feel silly. Fixes #1144

[codecov-io]

Codecov Report

Merging #1185 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1185      +/-   ##
==========================================
+ Coverage   91.62%   91.63%   +<.01%     
==========================================
  Files         127      127              
  Lines        4563     4565       +2     
  Branches     1502     1502              
==========================================
+ Hits         4181     4183       +2     
  Misses        160      160              
  Partials      222      222

Impacted Files	Coverage Δ
src/generators/nodes/Slot.ts	90.16% <100%> (+0.33%)	⬆️

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