Fix Plot with no variance in orient dimension

[mwaskom]

Fixes #3071

[codecov]

Codecov Report

Merging #3084 (16dcd28) into master (876bedd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3084   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files          74       74           
  Lines       23749    23759   +10     
=======================================
+ Hits        23368    23378   +10     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_core/scales.py	89.74% <100.00%> (+0.07%)	⬆️
tests/_core/test_plot.py	98.66% <100.00%> (+<0.01%)	⬆️
tests/_marks/test_line.py	100.00% <100.00%> (ø)