Fix empty country code stored in consent records

When IP geolocation returned no matching CIDR block, LookupCountryByIP
returned an empty string with nil error. The handler took the address of
that empty string, producing a non-nil pointer to "", which was inserted
into the database. Guard against this by returning nil when the resolved
country code is empty, and backfill existing rows with a migration.

Signed-off-by: Émile Ré <emile@getprobo.com>

pkg/coredata/migrations/20260511T160400Z.sql

@@ -0,0 +1,17 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+UPDATE cookie_consent_records
+SET country_code = NULL
+WHERE country_code = '';

pkg/server/api/cookiebanner/v1/handler.go

@@ -108,6 +108,10 @@ func (h *Handler) resolveCountryCode(r *http.Request) *coredata.CountryCode {
 		return nil
 	}
 
+	if cc == "" {
+		return nil
+	}
+
 	return &cc
 }