[202205] Fix exception in adding mirror_session when gre_type is absent

[bingwang-ms]

What I did

This PR is a duplication of https://github.com/sonic-net/sonic-utilities/pull/2458/files.
This PR is to fix the exception in adding mirror session when gre_type is absent.

~$ sudo config mirror_session add session_1 25.25.25.25 10.1.1.1 8 100
Traceback (most recent call last):
  File "/usr/local/bin/config", line 8, in <module>
    sys.exit(config())
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1135, in invoke
    sub_ctx = cmd.make_context(cmd_name, args, parent=ctx)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 641, in make_context
    self.parse_args(ctx, args)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 940, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 1476, in handle_parse_result
    value = invoke_param_callback(
  File "/usr/local/lib/python3.9/dist-packages/click/core.py", line 96, in invoke_param_callback
    return callback(ctx, param, value)
  File "/usr/local/lib/python3.9/dist-packages/config/main.py", line 1069, in validate_gre_type
    if value.lower().startswith('0x'):
AttributeError: 'NoneType' object has no attribute 'lower'

How I did it

Add a check in validate_gre_type. If gre_type is absent, return None directly.

How to verify it

Verified by UT and running on a testbed.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)