UnicodePlots: do not show blank plots (layouts)

[t-bltg]

Goes with JuliaPlots/UnicodePlots.jl#212.

[codecov]

Codecov Report

Merging #4058 (7a6c275) into master (acb5d5a) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4058      +/-   ##
==========================================
+ Coverage   62.13%   62.15%   +0.02%     
==========================================
  Files          29       29              
  Lines        7286     7288       +2     
==========================================
+ Hits         4527     4530       +3     
+ Misses       2759     2758       -1     

Impacted Files	Coverage Δ
src/backends/unicodeplots.jl	51.74% <100.00%> (+0.68%)	⬆️
src/args.jl	72.10% <0.00%> (ø)
src/backends/gr.jl	88.36% <0.00%> (+0.08%)	⬆️

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