[CMIS]The "get_error_description" function should return 'OK' instead…

… of None when there are no errors. (#528)

<!-- Provide a general summary of your changes in the Title above -->

#### Description

The "get_error_description" function should return 'OK' instead of 'None' when there are no errors.

#### Motivation and Context
When there are no errors on the cable, get_error_description currently returns 'None'. In this case, we should return 'OK' so that the CLI (show interface transceiver error-description) provides a proper output.

#### How Has This Been Tested?
Call this API directly on different cables with different error statuses.

#### Additional Information (Optional)

sonic_platform_base/sonic_xcvr/api/public/cmis.py

@@ -3152,6 +3152,6 @@ def get_error_description(self):
         if state != CmisCodes.MODULE_STATE[3]:
             return state
 
-        return None
+        return 'OK'
 
     # TODO: other XcvrApi methods

tests/sonic_xcvr/test_cmis.py

@@ -3066,7 +3066,7 @@ def test_get_error_description(self):
         self.api.xcvr_eeprom.read.return_value = 0x10
 
         result = self.api.get_error_description()
-        assert result is None
+        assert result is 'OK'
 
     def test_random_read_fail(self):
         def mock_read_raw(offset, size):