don't fail on users without signature(s)

[dmelha]

Signed-off-by: dmelha dmelha@localhost

extra check for type dict added because users without signature(s) can have a list object there.
example output without patch:

python3 webapp_admin.py.unpatched.py --all-users --backup --backup-signature
Creating backup WebApp settings for user kkopanocek
Dumping: 'Neue Signatur' to './kkopanocek_Neue-Signatur_1551091094578.html' 
konferenzraum: Has no or no valid WebApp settings creating empty config tree
Creating backup WebApp settings for user konferenzraum
konferenzraum: Has no or no valid WebApp settings creating empty config tree
user konferenzraum has no signature
Creating backup WebApp settings for user supertest100
Traceback (most recent call last):
  File "webapp_admin.py.unpatched.py", line 584, in <module>
    main()
  File "webapp_admin.py.unpatched.py", line 518, in main
    backup_signature(user, options.location)
  File "webapp_admin.py.unpatched.py", line 217, in backup_signature
    if settings['settings']['zarafa']['v1']['contexts']['mail'].get('signatures'):
AttributeError: 'list' object has no attribute 'get'

[dmelha]

any feedback?