Skip disks information from agent if it's empty

Sometimes our discovery agent sends an empty `disks` array for a long time.
This leads to the fact that a user gets a lot of notifications in UI.

As a quick fix, we can ignore an empty `disks` array if a node already has
a valid disks information.

Closes-Bug: #1278774
Change-Id: I7dd2ed43fbb3184c748fefce2dee72e46d666948
Merge "Skip disks information from agent if it's empty"