Fix deprecation warning when sorting data frame with no columns (#3190)

Author: bkamins

src/abstractdataframe/sort.jl

@@ -209,8 +209,10 @@ function ordering(df::AbstractDataFrame, cols::AbstractVector, lt::Function,
                   by::Function, rev::Bool, order::Ordering)
 
     if length(cols) == 0
-        Base.depwarn("When empty column selector is passed ordering is done on all colums. " *
-                     "This behavior is deprecated and will change in the future.", :ordering)
+        if ncol(df) > 0
+            Base.depwarn("When empty column selector is passed ordering is done on all colums. " *
+                        "This behavior is deprecated and will change in the future.", :ordering)
+        end
         return ordering(df, lt, by, rev, order)
     end
 

src/other/metadata.jl

@@ -1,7 +1,7 @@
 ### Metadata API from DataAPI.jl
 
-# private type that is passed as a default value in metadata and colmetadata
-# do detect the fact that no default was passed
+# private type that is passed as a default value in the metadata and colmetadata
+# functions to detect the fact that no default was passed
 struct MetadataMissingDefault end
 
 # DataAPI.metadatasupport and DataAPI.colmetadatasupport are not exported

test/sort.jl

@@ -10,6 +10,10 @@ using DataFrames, Random, Test, CategoricalArrays
 
     d = DataFrame(dv1=dv1, dv2=dv2, dv3=dv3, cv1=cv1)
 
+    @test sort(DataFrame()) == DataFrame()
+    @test sort!(DataFrame()) == DataFrame()
+    @test isempty(sortperm(DataFrame()))
+    @test issorted(DataFrame())
     @test sortperm(d) == sortperm(dv1)
     @test sortperm(d[:, [:dv3, :dv1]]) == sortperm(dv3)
     @test sort(d, :dv1)[!, :dv3] == sort(d, "dv1")[!, "dv3"] == sortperm(dv1)