Can't use empty vectors in `th_cheb2leg` or `th_leg2cheb`

[DanielVandH]

This originally came up in https://github.com/JuliaApproximation/ClassicalOrthogonalPolynomials.jl/issues/196

julia> FastTransforms.th_leg2cheb(Float64[])
ERROR: BoundsError: attempt to access 0×0 Matrix{Float64} at index [1, 1:0]
Stacktrace:
  [1] throw_boundserror(A::Matrix{Float64}, I::Tuple{Int64, Base.Slice{Base.OneTo{Int64}}})
    @ Base .\abstractarray.jl:737
  [2] checkbounds
    @ .\abstractarray.jl:702 [inlined]
  [3] view
    @ .\subarray.jl:184 [inlined]
  [4] maybeview
    @ .\views.jl:148 [inlined]
  [5] dotview
    @ .\broadcast.jl:1244 [inlined]
  [6] _leg2chebTH_TLC(::Type{Float64}, mn::Tuple{Int64}, d::Int64)
    @ FastTransforms C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:181
  [7] _broadcast_getindex_evalf
    @ .\broadcast.jl:709 [inlined]
  [8] _broadcast_getindex
    @ .\broadcast.jl:692 [inlined]
  [9] #31
    @ .\broadcast.jl:1118 [inlined]
 [10] ntuple
    @ .\ntuple.jl:48 [inlined]
 [11] copy
    @ .\broadcast.jl:1118 [inlined]
 [12] materialize
    @ .\broadcast.jl:903 [inlined]
 [13] plan_th_leg2cheb!
    @ C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:203 [inlined]
 [14] plan_th_leg2cheb!
    @ C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:651 [inlined]
 [15] th_leg2cheb(::Vector{Float64})
    @ FastTransforms C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:652
 [16] top-level scope
    @ REPL[20]:1

julia> FastTransforms.th_cheb2leg(Float64[])
ERROR: ArgumentError: invalid Array dimensions
Stacktrace:
  [1] Array
    @ .\boot.jl:477 [inlined]
  [2] Array
    @ .\boot.jl:486 [inlined]
  [3] zeros
    @ .\array.jl:636 [inlined]
  [4] zeros
    @ .\array.jl:632 [inlined]
  [5] _cheb2legTH_TLC(::Type{Float64}, mn::Tuple{Int64}, d::Int64)
    @ FastTransforms C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:218
  [6] _broadcast_getindex_evalf
    @ .\broadcast.jl:709 [inlined]
  [7] _broadcast_getindex
    @ .\broadcast.jl:692 [inlined]
  [8] #31
    @ .\broadcast.jl:1118 [inlined]
  [9] ntuple
    @ .\ntuple.jl:48 [inlined]
 [10] copy
    @ .\broadcast.jl:1118 [inlined]
 [11] materialize
    @ .\broadcast.jl:903 [inlined]
 [12] plan_th_cheb2leg!
    @ C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:235 [inlined]
 [13] plan_th_cheb2leg!
    @ C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:651 [inlined]
 [14] th_cheb2leg(::Vector{Float64})
    @ FastTransforms C:\Users\danjv\.julia\packages\FastTransforms\S7huq\src\toeplitzhankel.jl:652
 [15] top-level scope
    @ REPL[21]:1