Add default to known_length

This needs a default value. Otherwise it results in a stack overflow.

Author: Tokazama

src/ArrayInterface.jl

@@ -36,6 +36,7 @@ known_length(x) = known_length(typeof(x))
 known_length(::Type{<:NTuple{N,<:Any}}) where {N} = N
 known_length(::Type{<:NamedTuple{L}}) where {L} = length(L)
 known_length(::Type{T}) where {T<:Base.Slice} = known_length(parent_type(T))
+known_length(::Type{T}) where {T} = false
 
 """
     can_change_size(::Type{T}) -> Bool

test/runtests.jl

@@ -204,6 +204,7 @@ end
     @test ArrayInterface.known_length(1:2) == nothing
     @test ArrayInterface.known_length((1,)) == 1
     @test ArrayInterface.known_length((a=1,b=2)) == 2
+    @test ArrayInterface.known_length([]) == nothing
 end
 
 @testset "indices" begin