Allow #nulls_first and #nulls_last in PostgreSQL

When using PostgreSQL, it's useful to be able to specify NULLS FIRST and NULLS LAST on ordered columns. With this commit you can do that now, as in:

```ruby
User.arel_table[:first_name].desc.nulls_last
```

activerecord/lib/arel/nodes.rb

@@ -18,6 +18,7 @@ require "arel/nodes/false"
 # unary
 require "arel/nodes/unary"
 require "arel/nodes/grouping"
+require "arel/nodes/ordering"
 require "arel/nodes/ascending"
 require "arel/nodes/descending"
 require "arel/nodes/unqualified_column"

activerecord/lib/arel/nodes/ordering.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Arel # :nodoc: all
+  module Nodes
+    class Ordering < Unary
+      def nulls_first
+        NullsFirst.new(self)
+      end
+
+      def nulls_last
+        NullsLast.new(self)
+      end
+    end
+
+    class NullsFirst < Ordering; end
+    class NullsLast < Ordering; end
+  end
+end

activerecord/lib/arel/nodes/unary.rb

@@ -36,7 +36,6 @@ module Arel # :nodoc: all
       Offset
       On
       OptimizerHints
-      Ordering
       RollUp
     }.each do |name|
       const_set(name, Class.new(Unary))

activerecord/lib/arel/visitors/postgresql.rb

@@ -82,6 +82,16 @@ module Arel # :nodoc: all
           visit o.right, collector
         end
 
+        def visit_Arel_Nodes_NullsFirst(o, collector)
+          visit o.expr, collector
+          collector << " NULLS FIRST"
+        end
+
+        def visit_Arel_Nodes_NullsLast(o, collector)
+          visit o.expr, collector
+          collector << " NULLS LAST"
+        end
+
         # Used by Lateral visitor to enclose select queries in parentheses
         def grouping_parentheses(o, collector)
           if o.expr.is_a? Nodes::SelectStatement

activerecord/test/cases/arel/visitors/postgres_test.rb

@@ -315,6 +315,22 @@ module Arel
           _(sql).must_be_like %{ "users"."name" IS DISTINCT FROM NULL }
         end
       end
+
+      describe "Nodes::Ordering" do
+        it "should handle nulls first" do
+          test = Table.new(:users)[:first_name].desc.nulls_first
+          _(compile(test)).must_be_like %{
+            "users"."first_name" DESC NULLS FIRST
+          }
+        end
+
+        it "should handle nulls last" do
+          test = Table.new(:users)[:first_name].desc.nulls_last
+          _(compile(test)).must_be_like %{
+            "users"."first_name" DESC NULLS LAST
+          }
+        end
+      end
     end
   end
 end