getting blank image in output

[pdpsinghr]

import mapnik
m = mapnik.Map(1024, 768, '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs')
mapnik_xml_path = '/usr/share/renderd/example-map/xml/2024_05_20_07_amd.xml'
m.srs = '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs'
mapnik.load_map(m, mapnik_xml_path)
bbox = mapnik.Box2d(-122.4,37.8, -122.3, 37.9)
m.zoom_to_box(bbox)
img = mapnik.Image(1024, 768)
mapnik.render(m, img)
img.save('output_image.png')

this is my python code but in response i am getting blank image