Fix Bode plot with zero magnitude

[baggepinnen]

Closes #438

[codecov[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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
:x: 0.047
:heavy_check_mark: 0.011
:heavy_check_mark: 0.001
:heavy_check_mark: 0.001
:heavy_check_mark: 0.007
:heavy_check_mark: 0.003
:heavy_check_mark: 0.0
:warning: 0.028
:x: 0.06
:heavy_check_mark: 0.0
:warning: 0.017