Several plugins call msg_curse(args=None) with args defaulting to None,
but then access args attributes directly (e.g. args.diskio_iops) without
checking if args is None first, causing AttributeError in tests and any
caller that omits args.
Add `args and` guard before every args attribute access in msg_curse for:
- diskio (4 places)
- fs (2 places)
- network (6 places)
- processlist (7 places)
- processcount (1 place)
- system (1 place)
- load (1 place)
- containers (1 place)
- gpu (2 places)
Also fix TestDiskioPluginMsgCurse tests to call update_views() before
msg_curse() so views are populated before rendering.
Fixes#3429
