fix: cater for empty async call

[parthea]

This PR fixes the issue in the stack trace below seen in cl/618127799 which is similar to #791 but for the async client

            # Certain fields should be provided within the metadata header;
            # add these here.
            metadata = tuple(metadata) + (
                gapic_v1.routing_header.to_grpc_metadata((
>                   ("resource", request.resource),
                )),
            )
E           AttributeError: 'NoneType' object has no attribute 'resource'

[ohmayr]

Do we still need this j2 else case here?

[parthea]

Yes, the else case is needed when{% if method.input.ident.package != method.ident.package %} is false and also exists in the sync version here

gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/_client_macros.j2

Lines 106 to 113 in 2307345

	{% else %}
	# Minor optimization to avoid making a copy if the user passes
	# in a {{ method.input.ident }}.
	# There's no risk of modifying the input as we've already verified
	# there are no flattened fields.
	if not isinstance(request, {{ method.input.ident }}):
	request = {{ method.input.ident }}(request)
	{% endif %}{# different request package #}