support empty strings for routing headers

[daniel-sanche]

The bigtable team is requesting that we always pass the app_profile_id header, even if not set. This allows them to verify that each client does support the header, even if it's not being actively used in each request.

Currently, the gapic generator only accepts truthy headers:

        if request.app_profile_id:
            header_params["app_profile_id"] = request.app_profile_id

I think we should change that to if request.app_profile_id is not None, to accept empty string values, as is used when building the proto