Fix exception in adding mirror_session when gre_type is absent (#2458)

This PR is to fix the exception in adding mirror session when gre_type is absent.

Author: bingwang-ms

config/main.py

@@ -1143,6 +1143,8 @@ def validate_ipv4_address(ctx, param, ip_addr):
 def validate_gre_type(ctx, _, value):
     """A validator for validating input gre_type
     """
+    if value is None:
+        return None
     try:
         base = 10
         if value.lower().startswith('0x'):

tests/config_mirror_session_test.py

@@ -85,6 +85,12 @@ def test_mirror_session_add():
                 ["test_session", "100.1.1.1", "2.2.2.2", "8", "63", "0", "0"])
 
         mocked.assert_called_with("test_session", "100.1.1.1", "2.2.2.2", 8, 63, 0, 0, None)
+        
+        result = runner.invoke(
+                config.config.commands["mirror_session"].commands["add"],
+                ["test_session", "100.1.1.1", "2.2.2.2", "8", "63"])
+
+        mocked.assert_called_with("test_session", "100.1.1.1", "2.2.2.2", 8, 63, None, None, None)
 
 
 def test_mirror_session_erspan_add():