Fix subnet-up/down scripts being called with an empty SUBNET.

Commit 052ff8b2c598358d1c5febaa9f9f5fc5d384cfd3 contained a bug that causes
scripts to be called with an empty, or possibly corrupted SUBNET variable when
a Subnet is added or removed while the owner is still online. In router mode,
this normally does not happen, but in switch mode this is normal.