for netcdf, missingval=nothing when there is not Missing in the type (#308)

* netcdf should have missingval=nothing when there is not Missing in the type

* better way to test missing union

Author: rafaqz

src/sources/ncdatasets.jl

@@ -171,8 +171,7 @@ function _layermetadata(ds, keys)
     NamedTuple{map(Symbol, keys)}(dimtypes)
 end
 
-missingval(var::NCD.CFVariable) = missing
-missingval(ds::NCD.Dataset) = missing
+missingval(var::NCD.CFVariable{T}) where T = missing isa T ? missing : nothing
 
 function layerkeys(ds::NCD.Dataset)
     dimkeys = _dimkeys(ds)

test/sources/ncdatasets.jl

@@ -280,6 +280,15 @@ stackkeys = (
         end
     end
 
+    @testset "no missing value" begin
+        write("nomissing.nc", boolmask(ncarray) .* 1)
+        nomissing = Raster("nomissing.nc")
+        @test missingval(nomissing) == nothing
+        rm("nomissing.nc")
+        @test name(ncarray) == :tos
+    end
+
+
     @testset "show" begin
         sh = sprint(show, MIME("text/plain"), ncarray)
         # Test but don't lock this down too much