migrate to sound null safety

migrate to sound null safety

tried to strike a balance between auto migration and checking what
can actually get null, or improve that by promoting some variables.

Learnt that for class fields dart cannot promote nullable fields to
null-safe after a if (field == null) return 'early'; check, so we
need to introduce a intermediate varibable and check on that - a bit
of a bummer..

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>