Fix: Handle unset union map type (deepmap#650)

Without this check, an uninitialised (zero-value) `t.union` will get
serialised as `null`, which then overrides the map's contents, leading
to a panic:

  panic: assignment to entry in nil map

By instead only unmarshalling this when we receive a non-nil version of
the `t.union`, we can handle zero-value'd structs.

internal/test/components/components_test.go

@@ -202,3 +202,13 @@ func TestAnyOf(t *testing.T) {
 	assert.NoError(t, err)
 	assert.Equal(t, OneOfVariant5{Discriminator: "all", Id: 456}, v5)
 }
+
+func TestMarshalWhenNoUnionValueSet(t *testing.T) {
+	const expected = `{"one":null,"three":null,"two":null}`
+
+	var dst OneOfObject10
+
+	bytes, err := dst.MarshalJSON()
+	assert.Nil(t, err)
+	assert.Equal(t, expected, string(bytes))
+}

pkg/codegen/templates/union.tmpl

@@ -65,9 +65,11 @@
                 return nil, err
             }
             object := make(map[string]json.RawMessage)
-            err = json.Unmarshal(b, &object)
-            if err != nil {
+            if t.union != nil {
+              err = json.Unmarshal(b, &object)
+              if err != nil {
                 return nil, err
+              }
             }
             {{range .Schema.Properties}}
                 object["{{.JsonFieldName}}"], err = json.Marshal(t.{{.GoFieldName}})