Support explicit empty arrays

[Mattriks]

• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• Fixes creating empty plot #751
• Adds support for explicit empty arrays

Example

p1 = plot(x=[], y=[], Geom.point)
p2 = plot(x=[], y=[], Geom.line, Guide.xticks(ticks=[0,10]), Guide.yticks(ticks=[0,10]) )
hstack(p1, p2)

[codecov-io]

Codecov Report

Merging #1392 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1392      +/-   ##
==========================================
+ Coverage   90.26%   90.31%   +0.04%     
==========================================
  Files          39       39              
  Lines        4242     4242              
==========================================
+ Hits         3829     3831       +2     
+ Misses        413      411       -2

Impacted Files	Coverage Δ
src/coord.jl	91.91% <100%> (ø)	⬆️
src/statistics.jl	96.92% <100%> (ø)	⬆️
src/Gadfly.jl	79.11% <0%> (+0.49%)	⬆️

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[Mattriks]

This is ready to merge.