[Mellanox] Fix issue: failed to decode Json while there is no hwsku.json (#11436)

- Why I did it
Fix bug: pmon report error on start up because some SKUs do not have hwsku.json

- How I did it
If hwsku.json, do not extract RJ45 port information

- How to verify it
Manual test.
Unit test.

platform/mellanox/mlnx-platform-api/sonic_platform/chassis.py

@@ -111,7 +111,8 @@ class Chassis(ChassisBase):
         self.reboot_cause_initialized = False
 
         # Build the RJ45 port list from platform.json and hwsku.json
-        self.RJ45_port_list = extract_RJ45_ports_index()
+        self._RJ45_port_inited = False
+        self._RJ45_port_list = None
 
         logger.log_info("Chassis loaded successfully")
 
@@ -124,6 +125,13 @@ class Chassis(ChassisBase):
             if SFP.shared_sdk_handle:
                 deinitialize_sdk_handle(SFP.shared_sdk_handle)
 
+    @property
+    def RJ45_port_list(self):
+        if not self._RJ45_port_inited:
+            self._RJ45_port_list = extract_RJ45_ports_index()
+            self._RJ45_port_inited = True
+        return self._RJ45_port_list
+
     ##############################################
     # PSU methods
     ##############################################

platform/mellanox/mlnx-platform-api/sonic_platform/utils.py

@@ -239,9 +239,13 @@ def load_json_file(filename, log_func=logger.log_error):
 def extract_RJ45_ports_index():
     # Cross check 'platform.json' and 'hwsku.json' to extract the RJ45 port index if exists.
     hwsku_path = device_info.get_path_to_hwsku_dir()
+    hwsku_file = os.path.join(hwsku_path, HWSKU_JSON)
+    if not os.path.exists(hwsku_file):
+        # Platforms having no hwsku.json do not have RJ45 port
+        return None
+
     platform_file = device_info.get_path_to_port_config_file()
     platform_dict = load_json_file(platform_file)['interfaces']
-    hwsku_file = os.path.join(hwsku_path, HWSKU_JSON)
     hwsku_dict = load_json_file(hwsku_file)['interfaces']
     port_name_to_index_map_dict = {}
     RJ45_port_index_list = []

platform/mellanox/mlnx-platform-api/tests/test_utils.py

@@ -120,3 +120,8 @@ class TestUtils:
     def test_run_command(self):
         output = utils.run_command('ls')
         assert output
+
+    @mock.patch('sonic_py_common.device_info.get_path_to_hwsku_dir', mock.MagicMock(return_value='/tmp'))
+    def test_extract_RJ45_ports_index(self):
+        rj45_list = utils.extract_RJ45_ports_index()
+        assert rj45_list is None