support nesting in map struct

[erankor]

previously, missing keys with children were ignored in MapStruct marshal. this leads to missing values, and the code eventually panics in -
chunkMap[name] = layout.PagesToChunk(pages)
since pages is an empty slice in this case, and PagesToChunk assumes it has at least one element.

the fix is to push to the stack all the missing keys in the map -

• simple fields (without children) get a null value (as was done previously)
• nested fields (with children) get an empty map value

sample code that reproduces the issue -

type Object1 struct {
	ID     string   `parquet:"name=id, type=BYTE_ARRAY, convertedtype=UTF8"`
	Nested *Object2 `parquet:"name=nested"`
}

type Object2 struct {
	ID string `parquet:"name=id, type=BYTE_ARRAY, convertedtype=UTF8"`
}

func TestParquetNestingWithMap(t *testing.T) {
	r := require.New(t)

	buf := &bytes.Buffer{}
	pw, err := writer.NewParquetWriter(&parquetFileWriter{buf}, new(Object1), 2)
	r.NoError(err)

	item := map[string]any{
		"ID": "hello",
		// "Nested" implicitly initialized to nil
	}
	err = pw.Write(item)
	r.NoError(err)

	err = pw.WriteStop()
	r.NoError(err)
}