MethodError updating Observable linestyle

[yha]

Looks like linestyle can't be updated at all. Example:

style = Observable(:dot)
scene = lines(rand(5), linestyle = style)
display(scene)
style[] = :solid 
# MethodError: Cannot `convert` an object of type Nothing to an object of type Array{Float64,1}

If :dot and :solid are swapped in this example, there's a different error:

MethodError: convert(::Type{Union{}}, ::Array{Float64,1}) is ambiguous

[ffreyer]

You can't switch between :solid and another linestyle because they result in different shader code. You can switch between different patterns, e.g. :dot and :dash.

We could fix this by treating solid lines as dashed lines with infinite dash length. The patterned line code-path is significantly more expensive though. I've been thinking it might a good compromise to make nothing use the fast path and :solid be a pseudo-dashed line.
The change for this should actually be very simple - just replace this with Float64[0] (or maybe Float64[0, 1e30]) and document

Makie.jl/src/conversions.jl

Lines 1114 to 1115 in 2b5931e

	if ls === :solid
	return nothing

Related #3641