fix: queryRows returns [] not null for empty result sets

internal/mcp/server.go

@@ -850,6 +850,7 @@ func queryRows(ctx context.Context, pool *db.Pool, query string, args ...any) *m
 
 	cols := rows.FieldDescriptions()
 	var items []map[string]any
+	items = make([]map[string]any, 0)
 	for rows.Next() {
 		vals, err := rows.Values()
 		if err != nil {