Problem: netvision uses 0 as dfl

[vyskocilm]

Solution: use NULL as other subdrivers

cc @aquette

fixes comment in #279

[clepple]

From the developer guide:

If your commit is just a change to one component, such as the HCL, upsd or a specific driver, prefix your commit message in a way that matches similar commits. This helps when searching the repository or tracking down a regression.

The "Problem:" / "Solution:" format is probably more appropriate for an issue than for a commit message.

In this case, please prefix the commits with something like snmp-ups: or netvision-mib:

[vyskocilm]

@clepple renamed commit message