[sonic-cfggen] Allow cfggen to work on system without ports (#7999)

#### Why I did it
Allow cfggen to work on system without ports in platform.json or in port_config.ini

#### How I did it
Add json write of PORT section only if the dictionary that contains the ports is not empty.  

#### How to verify it
sonic-cfggen -k ACS-MSN3700 -H -j /etc/sonic/init_cfg.json --print-data

src/sonic-config-engine/portconfig.py

@@ -261,9 +261,9 @@ def parse_platform_json_file(hwsku_json_file, platform_json_file):
     port_dict = readJson(platform_json_file)
     hwsku_dict = readJson(hwsku_json_file)
 
-    if not port_dict:
+    if port_dict is None:
         raise Exception("port_dict is none")
-    if not hwsku_dict:
+    if hwsku_dict is None:
         raise Exception("hwsku_dict is none")
 
     if INTF_KEY not in port_dict or INTF_KEY not in  hwsku_dict:
@@ -285,8 +285,8 @@ def parse_platform_json_file(hwsku_json_file, platform_json_file):
 
         ports.update(child_ports)
 
-    if not ports:
-        raise Exception("Ports dictionary is empty")
+    if ports is None:
+        raise Exception("Ports dictionary is None")
 
     for i in ports.keys():
         port_alias_map[ports[i]["alias"]]= i

src/sonic-config-engine/sonic-cfggen

@@ -304,7 +304,7 @@ def main():
         if args.port_config is None:
             args.port_config = device_info.get_path_to_port_config_file(hwsku)
         (ports, _, _) = get_port_config(hwsku, platform, args.port_config, asic_id)
-        if not ports:
+        if ports is None:
             print('Failed to get port config', file=sys.stderr)
             sys.exit(1)
         deep_update(data, {'PORT': ports})

src/sonic-config-engine/tests/test_cfggen_platformJson.py

@@ -2,11 +2,17 @@ import ast
 import json
 import os
 import subprocess
+import sys
 
 import tests.common_utils as utils
 
 from unittest import TestCase
+from portconfig import get_port_config, INTF_KEY
 
+if sys.version_info.major == 3:
+    from unittest import mock
+else:
+    import mock
 
 # Global Variable
 PLATFORM_OUTPUT_FILE = "platform_output.json"
@@ -85,3 +91,10 @@ class TestCfgGenPlatformJson(TestCase):
         output_dict = ast.literal_eval(output.strip())
         expected = ast.literal_eval(json.dumps(fh_data))
         self.assertDictEqual(output_dict, expected)
+
+    @mock.patch('portconfig.readJson', mock.MagicMock(return_value={INTF_KEY:{}}))
+    @mock.patch('os.path.isfile', mock.MagicMock(return_value=True))
+    def test_platform_json_no_interfaces(self):
+        (ports, _, _) = get_port_config(port_config_file=self.platform_json)
+        self.assertNotEqual(ports, None)
+        self.assertEqual(ports, {})