Geom_ribbon: replace nan separator by missing

[Mattriks]

• I've run the regression tests (involving Geom.ribbon)
• I've built the docs and confirmed these changes don't cause new errors

This PR

• fixes InexactError when using integers in Geom.ribbon #1477
• The solution here is made possible by Missing and NaN support Compose.jl#397

[codecov-commenter]

Codecov Report

Merging #1478 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1478   +/-   ##
=======================================
  Coverage   89.17%   89.17%           
=======================================
  Files          39       39           
  Lines        4398     4398           
=======================================
  Hits         3922     3922           
  Misses        476      476           

Impacted Files	Coverage Δ
src/geom/ribbon.jl	100.00% <100.00%> (ø)

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