test: explicitly empty db_url must abort startup (PR #53 rework)

db_url: "" currently silently derives the state_dir-based sqlite URL,
which is a default applied to a SET value; state_dir: "" already
aborts. Failing test first, fix follows.

internal/config/config_validation_test.go

@@ -395,6 +395,25 @@ func TestExplicitNullValueAbortsStartup(t *testing.T) {
 	}
 }
 
+// TestExplicitlyEmptyDBURLAbortsStartup verifies that db_url set to an
+// empty string aborts startup: the derived file:...state.sqlite3 URL is
+// a default, and defaults apply only to omitted keys. This matches
+// state_dir, where an explicitly empty value already aborts.
+func TestExplicitlyEmptyDBURLAbortsStartup(t *testing.T) {
+	yamlContent := "signing_key: " + validTestSigningKey + "\ndb_url: \"\"\n"
+
+	c, err := configFromYAML(t, yamlContent)
+	if err == nil {
+		t.Fatalf("explicitly empty db_url must abort startup, got config: %+v", c)
+	}
+
+	t.Logf("got expected error: %v", err)
+
+	if !strings.Contains(err.Error(), "db_url") {
+		t.Errorf("error %q does not name the offending key db_url", err.Error())
+	}
+}
+
 func TestUnknownTopLevelKeyAbortsStartup(t *testing.T) {
 	yamlContent := `signing_key: ` + validTestSigningKey + `
 whitelist_hosts: