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/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
 }