d2format: Escape null correctly

d2compiler/compile_test.go

@@ -1656,7 +1656,7 @@ choo: {
 			text: `null
 `,
 			assertions: func(t *testing.T, g *d2graph.Graph) {
-				tassert.Equal(t, "null", g.Objects[0].ID)
+				tassert.Equal(t, "'null'", g.Objects[0].ID)
 				tassert.Equal(t, "null", g.Objects[0].IDVal)
 			},
 		},

d2format/escape.go

@@ -47,6 +47,10 @@ func escapeUnquotedValue(s string, inKey bool) string {
 		return `""`
 	}
 
+	if strings.EqualFold(s, "null") {
+		return `'null'`
+	}
+
 	var b strings.Builder
 	for i, r := range s {
 		switch r {

d2format/escape_test.go

@@ -170,7 +170,7 @@ func TestEscapeUnquoted(t *testing.T) {
 		{
 			name: "null",
 			str:  `null`,
-			exp:  `\null`,
+			exp:  `'null'`,
 		},
 		{
 			name: "empty",

testdata/d2compiler/TestCompile/null.exp.json

@@ -53,7 +53,7 @@
     "edges": null,
     "objects": [
       {
-        "id": "null",
+        "id": "'null'",
         "id_val": "null",
         "label_dimensions": {
           "width": 0,