inconsistent empty keys parse/print

[Roger-luo]

since there is no null type in TOML specification, people are supposed to use an empty field to represent this, but the TOML.print function does not implement this correctly, e.g

julia> s = """
       user = "me"
       [julia]
       [option]
       """
"user = \"me\"\n[julia]\n[option]\n"

julia> d = TOML.parse(s)
Dict{String,Any} with 3 entries:
  "julia"  => Dict{String,Any}()
  "option" => Dict{String,Any}()
  "user"   => "me"

julia> TOML.print(d) # incorrect
user = "me"

See also: toml-lang/toml#30