db-admin-relation-joined crash with gojuju

[Andreas Hasenack]

2013/05/10 23:13:11 INFO worker/uniter: HOOK Traceback (most recent call last):
2013/05/10 23:13:11 INFO worker/uniter: HOOK File "/var/lib/juju/agents/unit-postgresql-0/charm/hooks/db-admin-relation-joined", line 1885, in <module>
2013/05/10 23:13:11 INFO worker/uniter: HOOK raise SystemExit(main())
2013/05/10 23:13:11 INFO worker/uniter: HOOK File "/var/lib/juju/agents/unit-postgresql-0/charm/hooks/db-admin-relation-joined", line 1848, in main
2013/05/10 23:13:11 INFO worker/uniter: HOOK db_admin_relation_joined_changed(user, 'all')
2013/05/10 23:13:11 INFO worker/uniter: HOOK File "/var/lib/juju/agents/unit-postgresql-0/charm/hooks/db-admin-relation-joined", line 1206, in db_admin_relation_joined_changed
2013/05/10 23:13:11 INFO worker/uniter: HOOK generate_postgresql_hba(postgresql_hba)
2013/05/10 23:13:11 INFO worker/uniter: HOOK File "/var/lib/juju/agents/unit-postgresql-0/charm/hooks/db-admin-relation-joined", line 651, in generate_postgresql_hba
2013/05/10 23:13:11 INFO worker/uniter: HOOK for unit in relation_list(relid):
2013/05/10 23:13:11 INFO worker/uniter: HOOK TypeError: 'NoneType' object is not iterable
2013/05/10 23:13:11 ERROR worker/uniter: hook failed: exit status 1

I think pyjuju returned an empty string instead of the string "null". That breaks the relation_list() code so it doesn't return an empty list in that case, but the python value of None.

Branch attached, but only tested against gojuju.