… table

When opening the 'Open Vector Layer' dialog box in a directory with a
.gpkg without the optional metadata tables, the following error was
emitted on the console:
```
ERROR 1: In ExecuteSQL(): sqlite3_prepare_v2(
      SELECT
        ref.table_name, md.metadata, gc.geometry_type_name
      FROM
        gpkg_metadata_reference AS ref
      JOIN
        gpkg_metadata AS md ON md.id = ref.md_file_id
      LEFT JOIN
        gpkg_geometry_columns AS gc ON gc.table_name = ref.table_name
      WHERE
        md.md_standard_uri = 'http://mrcc.com/qgis.dtd'
        AND ref.reference_scope = 'table'
        AND md.md_scope = 'dataset'
      ):
  no such table: gpkg_metadata_reference
```