Prefetch: Remove "variables" field when no variables present

[jamilabreu]

Setting prefetch: true on a query with no variables sends a request with the following field

"variables" => true

Might it be better to either remove that field, or send an empty json object by default instead?

Using prefetch() { return {} } in my implementation at the moment.

cc: @benwilson512 for input on the absinthe-graphql side.

[benwilson512]

Yeah just to weigh in, this is the only instance where I've seen the "variables" param come in as something other than a json object or a string.