Add method value(::Nothing)

Author: simsurace

src/parameters_base.jl

@@ -18,3 +18,4 @@ value(x::AbstractArray) = map(value, x)
 value(x::Tuple) = map(value, x)
 value(x::NamedTuple) = map(value, x)
 value(x::Dict) = Dict(k => value(v) for (k, v) in x)
+value(::Nothing) = nothing

test/parameters.jl

@@ -62,7 +62,7 @@
     end
 
     @testset "value_flatten" begin
-        x = (ones(3), fixed(5.0), (a=fixed(5.0), b=[6.0, 2.1]))
+        x = (ones(3), fixed(5.0), (a=fixed(5.0), b=[6.0, 2.1]), nothing)
         v, unflatten = value_flatten(x)
 
         @test length(v) == 5