[FIX] core: post_load default value should be same as *_init_hook

-Step to reproduce: add a post_load method in the init of any module,
specify in the manifest like : 'post_load': 'post_load'. in v16 or above
Run
test_manifests of the test_lint module and we will get warning

closes odoo/odoo#156593

X-original-commit: f87473e82af8372346320ca747c1b41d3d5cf3c5
Signed-off-by: Julien Castiaux (juc) <juc@odoo.com>

odoo/addons/base/tests/test_module.py

@@ -60,7 +60,7 @@ class TestModuleManifest(BaseCase):
             'name': f'Temp {self.module_name}',
             'new_page_templates': {},
             'post_init_hook': '',
-            'post_load': None,
+            'post_load': '',
             'pre_init_hook': '',
             'sequence': 100,
             'summary': '',

odoo/modules/module.py

@@ -50,7 +50,7 @@ _DEFAULT_MANIFEST = {
     'new_page_templates': {},  # website themes
     #name, mandatory
     'post_init_hook': '',
-    'post_load': None,
+    'post_load': '',
     'pre_init_hook': '',
     'sequence': 100,
     'summary': '',