fix(yaml): emit null, not empty scalars

This is the null value the users expect.
It was how Remarshal worked with PyYAML.

The choice of how to implement it
is per the suggestion of the developer of ruamel.yaml
on https://stackoverflow.com/questions/44313992/.

Author: dbohdan

src/remarshal/main.py

@@ -646,6 +646,10 @@ def stringify_null(x: Any) -> Any:
         raise ValueError(msg)
 
 
+def _yaml_represent_none(self, data):
+    return self.represent_scalar("tag:yaml.org,2002:null", "null")
+
+
 def _encode_yaml(data: Document, *, yaml_options: YAMLOptions) -> str:
     yaml = ruamel.yaml.YAML()
     yaml.default_flow_style = False
@@ -654,6 +658,8 @@ def _encode_yaml(data: Document, *, yaml_options: YAMLOptions) -> str:
     yaml.indent = yaml_options.indent
     yaml.width = yaml_options.width
 
+    yaml.representer.add_representer(type(None), _yaml_represent_none)
+
     try:
         out = StringIO()
         yaml.dump(

tests/bool-null-key.yaml

@@ -1,4 +1,4 @@
 True: foo
 false: oof
 another: bar
-NULL: "nothin'"
+NULL: nothin'

tests/null.json

@@ -0,0 +1,3 @@
+{
+    "foo": null
+}

tests/null.yaml

@@ -0,0 +1 @@
+foo: null

tests/test_remarshal.py

@@ -1,6 +1,6 @@
 #! /usr/bin/env python
 # Remarshal, a utility to convert between serialization formats.
-# Copyright (c) 2014-2020, 2023 D. Bohdan
+# Copyright (c) 2014-2020, 2023-2024 D. Bohdan
 # License: MIT
 
 from __future__ import annotations
@@ -364,6 +364,11 @@ def test_binary_to_yaml(self, convert_and_read) -> None:
     def test_binary_to_cbor(self, convert_and_read) -> None:
         convert_and_read("bin.msgpack", "msgpack", "cbor")
 
+    def test_yaml_null(self, convert_and_read) -> None:
+        output = convert_and_read("null.json", "json", "yaml")
+        reference = read_file("null.yaml")
+        assert output == reference
+
     def test_yaml_style_default(self, convert_and_read) -> None:
         output = convert_and_read("long-line.json", "json", "yaml")
         reference = read_file("long-line-default.yaml")
@@ -544,6 +549,16 @@ def test_yaml2toml_bool_null_key(self, convert_and_read) -> None:
         reference = read_file("bool-null-key.toml")
         assert output == reference
 
+    def test_yaml2yaml_bool_null_key(self, convert_and_read) -> None:
+        output = convert_and_read(
+            "bool-null-key.yaml",
+            "yaml",
+            "yaml",
+        )
+        reference = read_file("bool-null-key.yaml")
+
+        assert output == reference.lower()
+
     def test_yaml2toml_timestamp_key(self, convert_and_read) -> None:
         output = convert_and_read(
             "timestamp-key.yaml",