Describe the bug
When executing following command with valid parameters
az network express-route gateway connection create --gateway-name --name --peering --resource-group

To Reproduce
az network express-route gateway connection create --gateway-name --name --peering --resource-group (basically anything without the optional, preview arugment: --propagated-route-tables) so the method create_express_route_connection is called with the default Parameter propagated_route_tables=None

Expected behavior
Create a new ExpressRoute Gateway connection

Environment summary
Install Method: MSI / Windows &
Azure Cloud Shell

Additional context
'NoneType' object is not iterable
Traceback (most recent call last):
File "/opt/az/lib/python3.6/site-packages/knack/cli.py", line 215, in invoke
cmd_result = self.invocation.execute(args)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 659, in execute
raise ex
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 722, in _run_jobs_serially
results.append(self._run_job(expanded_arg, cmd_copy))
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 715, in _run_job
six.reraise(*sys.exc_info())
File "/opt/az/lib/python3.6/site-packages/six.py", line 703, in reraise
raise value
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 693, in _run_job
result = cmd_copy(params)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/commands/init.py", line 330, in call
return self.handler(*args, **kwargs)
File "/opt/az/lib/python3.6/site-packages/azure/cli/core/init.py", line 784, in default_command_handler
return op(**command_args)
File "/opt/az/lib/python3.6/site-packages/azure/cli/command_modules/network/custom.py", line 2654, in create_express_route_connection
ids=[SubResource(id=propagated_route_table) for propagated_route_table in propagated_route_tables]
TypeError: 'NoneType' object is not iterable