limits tries to use default even when passed nothing

[aplavin]

# works:
julia> Axis(Figure()[1,1], limits=((0, nothing), nothing))

# errors:
julia> Axis(Figure()[1,1], limits=((1000, nothing), nothing))
ERROR: Invalid x-limits as xlims[1] <= xlims[2] is not met for (1000, 10.0).

As @jkrumbiegel said on slack,

it still takes the 10 from the default 0 to 10

• what version of Makie are you running? (]st -m Makie) v0.21
• can you reproduce the bug with a fresh environment ? (]activate --temp; add Makie)
• What platform + GPU are you on? any