`/createRoom`: "ValueError: invalid hostname:" when sending empty/invalid 3pid invite

This issue has been migrated from [#13511](https://github.com/matrix-org/synapse/issues/13511).

---

### Description

When sending:

```
{"name":"a","invite_3pid":[{"address":"","id_access_token":"","id_server":"","medium":""}]}
```

to `/_matrix/client/v3/createRoom` the server returns an internal server (see log below)

### Steps to reproduce

- Create a room and set the 3pid invite to an address of an empty string. You get:

```
{
"errcode": "M_UNKNOWN",
"error": "Internal server error"
}
```

### Homeserver

localhost

### Synapse Version

{"server_version":"1.64.0","python_version":"3.9.13"}

### Installation Method

Docker (matrixdotorg/synapse)

### Platform

Kubernetes with postgres and the official docker image

### Relevant log output

```shell
2022-08-11 22:26:00,473 - synapse.http.client - 455 - INFO - POST-4392- Error sending request to GET https:///_matrix/identity/v2/hash_details?access_token=: ValueError invalid hostname:
2022-08-11 22:26:00,475 - synapse.handlers.identity - 632 - WARNING - POST-4392- Error when looking up hashing details: invalid hostname:
2022-08-11 22:26:00,481 - synapse.http.client - 455 - INFO - POST-4392- Error sending request to POST https:///_matrix/identity/api/v1/store-invite: ValueError invalid hostname:
2022-08-11 22:26:00,481 - synapse.http.server - 183 - ERROR - POST-4392- Failed handle request via 'RoomCreateRestServlet':
Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 366, in _async_render_wrapper
callback_return = await self._async_render(request)
File "/usr/local/lib/python3.9/site-packages/synapse/http/server.py", line 572, in _async_render
callback_return = await raw_callback_return
File "/usr/local/lib/python3.9/site-packages/synapse/rest/client/room.py", line 91, in on_POST
info, _ = await self._room_creation_handler.create_room(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room.py", line 990, in create_room
) = await self.hs.get_room_member_handler().do_3pid_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room_member.py", line 1469, in do_3pid_invite
event, stream_id = await self._make_and_store_3pid_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/room_member.py", line 1553, in _make_and_store_3pid_invite
) = await self.identity_handler.ask_id_server_for_third_party_invite(
File "/usr/local/lib/python3.9/site-packages/synapse/handlers/identity.py", line 882, in ask_id_server_for_third_party_invite
data = await self.blacklisting_http_client.post_json_get_json(
File "/usr/local/lib/python3.9/site-packages/synapse/http/client.py", line 551, in post_json_get_json
response = await self.request(
File "/usr/local/lib/python3.9/site-packages/synapse/http/client.py", line 443, in request
response = await make_deferred_yieldable(request_deferred)
ValueError: invalid hostname:
```

### Anything else that would be useful to know?

_No response_

Start at synapse/rest/client/room.py and trace RoomCreateRestServlet through create_room into synapse/handlers/room_member.py and synapse/handlers/identity.py. Reproduce the request with the empty invite payload and follow the existing traceback; done means invalid 3pid invite data no longer produces an internal-server error or the invalid-hostname failure.

Written by the indexing model from the issue text.