Allow constructing Matrix from empty dataframe (#2878)

Author: jakobnissen

src/abstractdataframe/abstractdataframe.jl

@@ -1158,7 +1158,7 @@ _filter!_helper_astable(df::AbstractDataFrame, nti::Tables.NamedTupleIterator, f
     delete!(df, _findall((x -> !(f(x)::Bool)).(nti)))
 
 function Base.Matrix(df::AbstractDataFrame)
-    T = reduce(promote_type, (eltype(v) for v in eachcol(df)))
+    T = reduce(promote_type, (eltype(v) for v in eachcol(df)), init=Union{})
     return Matrix{T}(df)
 end
 

test/conversions.jl

@@ -6,6 +6,8 @@ const ≅ = isequal
 
 @testset "Constructors to Base types" begin
     df = DataFrame()
+    @test Matrix(df) isa Matrix{Union{}}
+    @test size(Matrix(df)) == (0, 0)
     df[!, :A] = 1:5
     df[!, :B] = 1.0:5.0
     @test isa(Matrix(df), Matrix{Float64})