Add assertions order by field with empty data

Add assertions to MySQL `ORDER BY FIELD()` with empty data.
These tests examine to sanitize `ORDER BY FIELD()` with empty data
appropriately.

```ruby
  Tag.order(['field(id, ?)', []]).to_sql
  # => SELECT "tags".* FROM "tags" ORDER BY field(id, NULL)

  Tag.order(['field(id, ?)', nil]).to_sql
  # => SELECT "tags".* FROM "tags" ORDER BY field(id, NULL)
```

activerecord/test/cases/relations_test.rb

@@ -358,6 +358,12 @@ def test_finding_with_complex_order
   def test_finding_with_sanitized_order
     query = Tag.order(["field(id, ?)", [1,3,2]]).to_sql
     assert_match(/field\(id, 1,3,2\)/, query)
+
+    query = Tag.order(["field(id, ?)", []]).to_sql
+    assert_match(/field\(id, NULL\)/, query)
+
+    query = Tag.order(["field(id, ?)", nil]).to_sql
+    assert_match(/field\(id, NULL\)/, query)
   end
 
   def test_finding_with_order_limit_and_offset