Fix layout engine parameterization when using Plot.on

[mwaskom]

Fixes #3211

The intended behavior here was to avoid overriding the user's choice of layout engine when they provide a matplotlib object with Plot.on, but None was getting passed through to Figure.set_layout_engine which unsets the user's choice (and defers to the rc defaults).

[codecov]

Codecov Report

Merging #3216 (f01ea33) into master (557b0d2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3216   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files          77       77           
  Lines       24347    24361   +14     
=======================================
+ Hits        23963    23977   +14     
  Misses        384      384           

Impacted Files	Coverage Δ
seaborn/_compat.py	48.03% <100.00%> (+1.57%)	⬆️
seaborn/_core/plot.py	99.20% <100.00%> (+<0.01%)	⬆️
tests/_core/test_plot.py	98.72% <100.00%> (+<0.01%)	⬆️