use correct gain type in zpk with length 0 numerator

[baggepinnen]

close #869

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (4f6bdba) 92.72% compared to head (ce4f79f) 92.72%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #870   +/-   ##
=======================================
  Coverage   92.72%   92.72%           
=======================================
  Files           5        5           
  Lines         330      330           
=======================================
  Hits          306      306           
  Misses         24       24           

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[JuliaControlBot]

This is an automated message.
Plots were compared to references. 11/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
❌ 0.047
✔️ 0.011
✔️ 0.001
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✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017