NodeMonitor out of disk space message is wrong for master

When the master goes out of disk space, it is put offline by the hudson.node_monitors.AbstractDiskSpaceMonitor system.

The message has a blank where the node name should be:

Putting back online as there is enough disk space again

I think this is just because c.getName() returns an empty string in https://github.com/jenkinsci/jenkins/blob/9d61a9a13171c94e55779d166d81599cdd0f9cb7/core/src/main/java/hudson/node_monitors/AbstractDiskSpaceMonitor.java#L43-L71.

Acceptance criteria

• This should show master instead of a space as the node name, i.e.

Putting back master online as there is enough disk space