Fix legendelements when children have no elements

[jkrumbiegel]

Description

Making a legend for a labeled histogram would fail because the text plot inside the hist would not return a legend element. Now that function is allowed to return an empty vector, it only fails afterwards if the combination of all subplots is still emtpy.

Type of change

Delete options that do not apply:

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Added unit tests for new algorithms, conversion methods, etc.

[MakieBot]

Compile Times benchmark

Note, that these numbers may fluctuate on the CI servers, so take them with a grain of salt. All benchmark results are based on the mean time and negative percent mean faster than the base branch. Note, that GLMakie + WGLMakie run on an emulated GPU, so the runtime benchmark is much slower. Results are from running:

using_time = @ctime using Backend
# Compile time
create_time = @ctime fig = scatter(1:4; color=1:4, colormap=:turbo, markersize=20, visible=true)
display_time = @ctime Makie.colorbuffer(display(fig))
# Runtime
create_time = @benchmark fig = scatter(1:4; color=1:4, colormap=:turbo, markersize=20, visible=true)
display_time = @benchmark Makie.colorbuffer(display(fig))

	using	create	display	create	display
GLMakie	8.80s (8.71, 8.95) 0.09+-	1.06s (1.03, 1.10) 0.03+-	781.95ms (752.92, 812.16) 21.47+-	9.40ms (9.33, 9.49) 0.06+-	86.42ms (85.74, 87.02) 0.46+-
master	8.83s (8.70, 8.93) 0.08+-	1.09s (1.06, 1.15) 0.03+-	769.32ms (761.76, 782.30) 6.28+-	9.37ms (9.25, 9.49) 0.08+-	85.95ms (85.57, 86.35) 0.28+-
evaluation	-0.30%, -0.03s invariant (-0.32d, 0.56p, 0.08std)	-2.90%, -0.03s invariant (-1.10d, 0.06p, 0.03std)	+1.61%, 12.62ms invariant (0.80d, 0.18p, 13.88std)	+0.34%, 0.03ms invariant (0.43d, 0.43p, 0.07std)	+0.54%, 0.47ms slower X (1.23d, 0.04p, 0.37std)
CairoMakie	10.03s (9.56, 11.16) 0.54+-	1.43s (1.38, 1.51) 0.04+-	316.52ms (270.20, 341.23) 24.33+-	14.74ms (14.49, 15.08) 0.18+-	7.55ms (7.32, 7.83) 0.15+-
master	10.06s (9.65, 10.63) 0.31+-	1.41s (1.36, 1.49) 0.05+-	325.26ms (308.41, 336.88) 11.10+-	14.78ms (14.36, 15.02) 0.21+-	7.64ms (7.41, 8.04) 0.22+-
evaluation	-0.34%, -0.03s invariant (-0.08d, 0.89p, 0.43std)	+0.95%, 0.01s invariant (0.30d, 0.58p, 0.04std)	-2.76%, -8.74ms invariant (-0.46d, 0.41p, 17.71std)	-0.25%, -0.04ms invariant (-0.18d, 0.74p, 0.20std)	-1.14%, -0.09ms invariant (-0.45d, 0.41p, 0.19std)
WGLMakie	10.78s (10.64, 10.89) 0.09+-	1.32s (1.29, 1.34) 0.02+-	12.01s (11.79, 12.17) 0.12+-	14.08ms (13.69, 14.67) 0.32+-	890.81ms (836.70, 992.81) 52.35+-
master	10.78s (10.58, 10.91) 0.11+-	1.31s (1.27, 1.35) 0.03+-	11.97s (11.77, 12.13) 0.14+-	14.28ms (13.39, 14.93) 0.51+-	878.85ms (823.69, 996.92) 60.47+-
evaluation	-0.02%, -0.0s invariant (-0.02d, 0.96p, 0.10std)	+0.25%, 0.0s invariant (0.14d, 0.80p, 0.02std)	+0.29%, 0.04s invariant (0.27d, 0.62p, 0.13std)	-1.39%, -0.2ms invariant (-0.46d, 0.41p, 0.42std)	+1.34%, 11.96ms invariant (0.21d, 0.70p, 56.41std)