Fix Bode plot with zero magnitude

[baggepinnen]

Closes #438

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e8f68d6) 92.87% compared to head (5360a25) 92.83%.

Files	Patch %	Lines
lib/ControlSystemsBase/src/plotting.jl	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
- Coverage   92.87%   92.83%   -0.04%     
==========================================
  Files          43       43              
  Lines        4813     4815       +2     
==========================================
  Hits         4470     4470              
- Misses        343      345       +2     

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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
✔️ 0.001
✔️ 0.007
✔️ 0.003
✔️ 0.0
⚠️ 0.028
❌ 0.06
✔️ 0.0
⚠️ 0.017