Wrong creation of empty polygon/polytope

[schillic]

# wrong
HPolytope{N}() where {N<:Real} = HPolytope{N}(Vector{N}(0))

# right
HPolytope{N}() where {N<:Real} = HPolytope{N}(Vector{LinearConstraint{N}}(0))

The old code works, probably because Julia internally converts, but needs one more allocation.

# wrong
julia> @time h = HPolytope{Int}()
  0.000064 seconds (7 allocations: 336 bytes)
LazySets.HPolytope{Int64}(LazySets.HalfSpace{Int64}[])

# right
julia> @time h = HPolytope{Int}()
  0.000064 seconds (6 allocations: 256 bytes)
LazySets.HPolytope{Int64}(LazySets.HalfSpace{Int64}[])