subdomains: Fix realm=None case for is_static_or_current_realm_url.
subdomains: Fix realm=None case for is_static_or_current_realm_url.

Fixes #22636.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip,zulip/zulip