StdVector loses type information if given empty vector

[glennmoy]

I think the root issue is that broadcasting CxxRef over an empty vector loses the type info

MWE using StdString, but in practice it's any user-defined wrapped C++ type

julia> StdVector{StdString}()
0-element CxxWrap.StdLib.StdVectorAllocated{StdString}

julia> StdVector(StdString[])
0-element CxxWrap.StdLib.StdVectorAllocated{Any}

julia> CxxRef.(StdString[])
Any[]

It may be enough to just check for isempty(v) before calling CxxRef here

function StdVector(v::Vector{T}) where {T}
  if (CxxWrapCore.cpp_trait_type(T) == CxxWrapCore.IsCxxType)
    return StdVector(CxxRef.(v))
  end
  result = StdVector{T}()
  append(result, v)
  return result
end

[omus]

Relatedly it would be good to also support StdVector{T}(::Vector):

julia> StdVector{StdString}()
0-element CxxWrap.StdLib.StdVectorAllocated{StdString}

julia> StdVector(StdString["hello"])
1-element CxxWrap.StdLib.StdVectorAllocated{StdString}:
 "hello"

julia> StdVector{StdString}(StdString["hello"])
ERROR: MethodError: no method matching StdVector{StdString}(::Vector{StdString})

Closest candidates are:
  StdVector{StdString}()
   @ CxxWrap ~/.julia/packages/CxxWrap/aXNBY/src/CxxWrap.jl:624

Stacktrace:
 [1] top-level scope
   @ REPL[18]:1