Fix dump in the presence of nothing

Currently anything that's `dump`ed that contains `nothing` will error,
since there's no logic in place for handling `nothing` in `dump` after
PR #27829.

base/show.jl

@@ -1613,11 +1613,12 @@ function dump(io::IOContext, @nospecialize(x), n::Int, indent)
     nothing
 end
 
-dump(io::IOContext, x::Module, n::Int, indent) = print(io, "Module ", x)
-dump(io::IOContext, x::String, n::Int, indent) = (print(io, "String "); show(io, x))
-dump(io::IOContext, x::Symbol, n::Int, indent) = print(io, typeof(x), " ", x)
-dump(io::IOContext, x::Union,  n::Int, indent) = print(io, x)
-dump(io::IOContext, x::Ptr,    n::Int, indent) = print(io, x)
+dump(io::IOContext, x::Module,  n::Int, indent) = print(io, "Module ", x)
+dump(io::IOContext, x::String,  n::Int, indent) = (print(io, "String "); show(io, x))
+dump(io::IOContext, x::Symbol,  n::Int, indent) = print(io, typeof(x), " ", x)
+dump(io::IOContext, x::Union,   n::Int, indent) = print(io, x)
+dump(io::IOContext, x::Ptr,     n::Int, indent) = print(io, x)
+dump(io::IOContext, x::Nothing, n::Int, indent) = show(io, x)
 
 function dump_elts(io::IOContext, x::Array, n::Int, indent, i0, i1)
     for i in i0:i1

test/show.jl

@@ -820,6 +820,9 @@ end
 let repr = sprint(dump, Test)
     @test repr == "Module Test\n"
 end
+let repr = sprint(dump, nothing)
+    @test repr == "nothing\n"
+end
 let a = Vector{Any}(undef, 10000)
     a[2] = "elemA"
     a[4] = "elemB"