Unable to use null as a preset in argument/field presets.

[TristenHarr]

Currently it's not possible to set argument or field presets as null, as you get the following error when building a supergraph if you write literal: null anywhere:

unable to parse metadata objects: invalid value: map, expected map with a single key

Originally posted on Discord here: https://discord.com/channels/407792526867693568/1300794264137236482