Don't crash in case of tables without signal field

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Don't crash in case of tables without alias field

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Don't crash in case of tables without metadata field

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>
Don't crash in case of tables without vmetadata field

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>