Fix dump in the presence of nothing

[ararslan]

Currently anything that's dumped that contains nothing will error, since there's no logic in place for handling nothing in dump after PR #27829, which removed print(::Nothing).

Before:

julia> dump(first(methods(sin)))
Method
  name: Symbol sin
  module: Module Base.MPFR
  file: Symbol mpfr.jl
  line: Int32 683
  sig: Tuple{typeof(sin),BigFloat} <: Any
  min_world: Int64 12691
  ambig: Nothing ERROR: ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.
Stacktrace:
 [1] print(::IOContext{Base.TTY}, ::Nothing) at ./show.jl:560
 [2] print(::IOContext{Base.TTY}, ::String, ::Nothing) at ./strings/io.jl:40
 [3] dump(::IOContext{Base.TTY}, ::Any, ::Int64, ::String) at ./show.jl:1611
 [4] dump(::IOContext{Base.TTY}, ::Any, ::Int64, ::String) at ./show.jl:1604
 [5] #dump#368 at ./show.jl:1688 [inlined]
 [6] (::getfield(Base, Symbol("#kw##dump")))(::NamedTuple{(:maxdepth,),Tuple{Int64}}, ::typeof(dump), ::IOContext{Base.TTY}, ::Method) at ./none:0
 [7] #dump#369(::Int64, ::Function, ::Method) at ./show.jl:1720
 [8] dump(::Method) at ./show.jl:1720
 [9] top-level scope at none:0

After:

julia> dump(first(methods(sin)))
Method
  name: Symbol sin
  module: Module Base.MPFR
  file: Symbol mpfr.jl
  line: Int32 683
  sig: Tuple{typeof(sin),BigFloat} <: Any
  min_world: Int64 12689
  ambig: Nothing nothing
  specializations: Nothing nothing
  sparam_syms: empty SimpleVector
  source: Array{UInt8}((579,)) UInt8[0x00, 0x03, 0x00, 0x00, 0x00, 0x23, 0x73, 0x65, 0x6c, 0x66  …  0xb7, 0xf6, 0x16, 0x01, 0xba, 0xf6, 0x00, 0x00, 0x10, 0x00]
  unspecialized: #undef
  generator: #undef
  roots: Array{Any}((3,))
    1: String "mpfr_sin"
    2: Ref{BigFloat} <: Any
    3: String "NaN result for non-NaN input."
  invokes: #undef
  nargs: Int32 2
  called: Int32 0
  nospecialize: Int32 0
  isva: Bool false
  pure: Bool false

[JeffBezanson]

Instead, line 1611 should be changed to call show by default. show is more correct for displaying an arbitrary unknown value.

[ararslan]

Fixed.

[ararslan]

FreeBSD and macOS failures are issues with Curl in the download test.