Fix viewer: constant columns with missing values

MWE:
julia> a = [missing, 1.0, 1.0, missing]
4-element Vector{Union{Missing, Float64}}:
  missing
 1.0
 1.0
  missing

julia> b = [missing, 1, missing, 1]
4-element Vector{Union{Missing, Int64}}:
  missing
 1
  missing
 1

julia> gtb = georef((; a, b, c = a * u"m", d = b * u"m"), CartesianGrid(2, 2))
4×5 GeoTable over 2×2 CartesianGrid{2,Float64}
┌────────────┬─────────────┬────────────┬─────────────┬─────────────────────────────────────────┐
│     a      │      b      │     c      │      d      │                geometry                 │
│ Continuous │ Categorical │ Continuous │ Categorical │               Quadrangle                │
│ [NoUnits]  │  [NoUnits]  │    [m]     │     [m]     │                                         │
├────────────┼─────────────┼────────────┼─────────────┼─────────────────────────────────────────┤
│  missing   │   missing   │  missing   │   missing   │ Quadrangle((0.0, 0.0), ..., (0.0, 1.0)) │
│    1.0     │      1      │   1.0 m    │     1 m     │ Quadrangle((1.0, 0.0), ..., (1.0, 1.0)) │
│    1.0     │   missing   │   1.0 m    │   missing   │ Quadrangle((0.0, 1.0), ..., (0.0, 2.0)) │
│  missing   │      1      │  missing   │     1 m     │ Quadrangle((1.0, 1.0), ..., (1.0, 2.0)) │
└────────────┴─────────────┴────────────┴─────────────┴─────────────────────────────────────────┘

julia> viewer(gtb)
ERROR: ArgumentError: range step cannot be zero
Stacktrace:
...
JuliaEarth / GeoStats.jl

Fix viewer: constant columns with missing values #335

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