Fix some minor errors in `dhcp_script`
Fix some minor errors in `dhcp_script`

1. Sometimes, `dnsmasq` will call `add` and `old` (i.e. the update
  routine) with precisely the same data, resulting in an empty `changes`
  dict.  This causes an invalid `UPDATE` call.

2. Defaulting the unset values to `None` will trigger an explicit `SET
  col=null` statement, however what we want to achieve is the default.
  Hence `text('DEFAULT')`.

3. `UpdatedAt` should actually be set.

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