julia create an empty dataframe and append rows to it

A zero length array defined using only [] will lack sufficient type information.

julia> typeof([])
Array{None,1}

So to avoid that problem is to simply indicate the type.

julia> typeof(Int64[])
Array{Int64,1}

And you can apply that to your DataFrame problem

julia> df = DataFrame(A = Int64[], B = Int64[])
0x2 DataFrame

julia> push!(df, [3  6])

julia> df
1x2 DataFrame
| Row | A | B |
|-----|---|---|
| 1   | 3 | 6 |