Empty NamedTuple definitions for get to appease JET

[quinnj]

Without these, we end up with JET errors like:

┌ get(f::StructUtils.var"#31#33", nt::@NamedTuple{}, key::Int64) @ Base ./namedtuple.jl:389
 │ invalid builtin function call: Base.getfield(nt::@NamedTuple{}, key::Int64)

I'm no expert in JET, and not sure if there are other simpler ways to fix this kind of thing, but it didn't seem like any assertion would work (can't assert that a NamedTuple has at least 1 key). Or because the isdefined on NamedTuple doesn't provide enough guarantee that nt is then definitely not empty? cc: @aviatesk

[JeffBezanson]

It seems like this should be fixed by improving analysis precision; we should know the getfield is not reachable. Also JET should have (I think it already has?) a no-false-positives mode, where it only gives errors like this if it can prove they are reachable.

[vtjnash]

It is perhaps just a general inference precision problem, that we return Bool instead of Const(false) when the object has no fields

julia> code_typed((Int,)) do x; isdefined(nothing, x); end
1-element Vector{Any}:
 CodeInfo(
    @ REPL[4]:1 within `#14`
1 ─ %1 =   builtin Main.isdefined(Main.nothing, x)::Bool
└──      return %1
) => Bool

[oscardssmith]

@vtjnash yeah looks like #58220 should fix it.

[quinnj]

Closing in favor of #58220