fix: make the home location lookup work against Claude

- Send the location question as a user message. A request with only a
  system message has no user content, which endpoints in front of Anthropic
  models reject ("text content blocks must be non-empty").
- Leave unknown fields out of the web search user_location. After a failed
  lookup there is no city or region, and the empty strings made every
  request fail ("user_location.city: String should have at least 1
  character").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_config_flow.py

@@ -1334,6 +1334,7 @@ async def test_subentry_web_search_user_location_lookup_fails(
         )
         await hass.async_block_till_done()
     assert mock_create.call_args.kwargs["model"] == "claude-sonnet-5-5"
+    assert mock_create.call_args.kwargs["input"][0]["role"] == "user"
     assert subentry_flow["type"] is FlowResultType.ABORT
     assert subentry_flow["reason"] == "reconfigure_successful"
     assert CONF_WEB_SEARCH_CITY not in subentry.data