Fix unitful axes with empty guides

[gustaphe]

Before UnitfulRecipes was merged into Plots, one could achieve an empty plot label for a unitful axis like this:

plot(randn(5)u"m"; yguide="")

Thanks to a bug which meant that you could plot unitful data on unitless axes, we didn't notice or care that this empty label actually didn't contain a unit at all.
After merging UnitfulRecipes into Plots, this bug was fixed, and this would error out:

plot(randn(5)u"m"; yguide="")
plot!(randn(5)u"m") # ERROR:  and m are not dimensionally compatible.

With this PR, that works.

[codecov]

Codecov Report

Base: 91.00% // Head: 91.00% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (3be7415) compared to base (8c7bf7e).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4537   +/-   ##
=======================================
  Coverage   91.00%   91.00%           
=======================================
  Files          40       40           
  Lines        7701     7702    +1     
=======================================
+ Hits         7008     7009    +1     
  Misses        693      693           

Impacted Files	Coverage Δ
src/unitful.jl	95.68% <100.00%> (+0.03%)	⬆️

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