Do not print anything related to extensions when they don't none exist

When extensions are supported but there's no one enabled in the
database, we should not print anything related to them in schema.rb.

activerecord/lib/active_record/schema_dumper.rb

@@ -70,11 +70,13 @@ def trailer(stream)
       def extensions(stream)
         return unless @connection.supports_extensions?
         extensions = @connection.extensions
-        stream.puts "  # These are extensions that must be enabled in order to support this database"
-        extensions.each do |extension|
-          stream.puts "  enable_extension #{extension.inspect}"
+        if extensions.any?
+          stream.puts "  # These are extensions that must be enabled in order to support this database"
+          extensions.each do |extension|
+            stream.puts "  enable_extension #{extension.inspect}"
+          end
+          stream.puts
         end
-        stream.puts
       end
 
       def tables(stream)

activerecord/test/cases/schema_dumper_test.rb

@@ -237,8 +237,18 @@ def test_schema_dump_includes_extensions
       unless connection.extension_enabled?('hstore')
         connection.enable_extension 'hstore'
       end
+
       output = standard_dump
+      assert_match "# These are extensions that must be enabled", output
       assert_match %r{enable_extension "hstore"}, output
+
+      connection.extensions.each do |ext|
+        connection.disable_extension ext
+      end
+
+      output = standard_dump
+      assert_no_match "# These are extensions that must be enabled", output
+      assert_no_match %r{enable_extension}, output
     end
 
     def test_schema_dump_includes_xml_shorthand_definition