feat: default node type for nothing

Author: MilesCranmer

src/Expression.jl

@@ -86,7 +86,7 @@ end
 end
 
 node_type(::Union{E,Type{E}}) where {N,E<:AbstractExpression{<:Any,N}} = N
-@unstable default_node_type(::Type{<:AbstractExpression}) = Node
+@unstable default_node_type(_) = Node
 default_node_type(::Type{<:AbstractExpression{T}}) where {T} = Node{T}
 
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