Fix "sort" filter on empty array to return empty array

When sorting an empty array with the "sort" filter, it returns nil
instead of [].  This confuses subsequent filters in the chain that
expect an array.  For example, when followed by the "map" filter, it
produces an array containing one nil element: [nil].

I could special-case the nil return value, but that would be more
cumbersome than making sure "sort" always returns an array.

Add a case to the "sort" method to return [] if the array is empty,
before performing any checks on ary.first that assume a non-empty array.

There is still a danger of returning nil if the first item in the array
is nil and it is non-empty, but I'm not sure how better to handle that
case.

Apply a similar fix to sort_natural, uniq, and compact filters.

test/integration/standard_filter_test.rb

@@ -158,6 +158,14 @@ class StandardFiltersTest < Minitest::Test
     assert_equal [{ "a" => 1 }, { "a" => 2 }, { "a" => 3 }, { "a" => 4 }], @filters.sort([{ "a" => 4 }, { "a" => 3 }, { "a" => 1 }, { "a" => 2 }], "a")
   end
 
+  def test_sort_empty_array
+    assert_equal [], @filters.sort([], "a")
+  end
+
+  def test_sort_natural_empty_array
+    assert_equal [], @filters.sort_natural([], "a")
+  end
+
   def test_legacy_sort_hash
     assert_equal [{ a: 1, b: 2 }], @filters.sort({ a: 1, b: 2 })
   end
@@ -177,6 +185,14 @@ class StandardFiltersTest < Minitest::Test
     assert_equal [testdrop], @filters.uniq([testdrop, TestDrop.new], 'test')
   end
 
+  def test_uniq_empty_array
+    assert_equal [], @filters.uniq([], "a")
+  end
+
+  def test_compact_empty_array
+    assert_equal [], @filters.compact([], "a")
+  end
+
   def test_reverse
     assert_equal [4, 3, 2, 1], @filters.reverse([1, 2, 3, 4])
   end