Add UpToDate key to kolide_softwareupdate tables when there are no recommended updates

[RebeccaMahany]

This should allow us to differentiate between no rows due to an error (e.g. a parsing error or a scanning error), and no rows due to the device being up-to-date. Now one row will be returned indicating that the device is up to date.

osquery> select * from kolide_softwareupdate_scan;
+------------+--------+----------+-------+-------+
| fullkey    | parent | key      | value | query |
+------------+--------+----------+-------+-------+
| 0/UpToDate | 0      | UpToDate | true  | *     |
+------------+--------+----------+-------+-------+
osquery> select * from kolide_softwareupdate;
+------------+--------+----------+-------+-------+
| fullkey    | parent | key      | value | query |
+------------+--------+----------+-------+-------+
| 0/UpToDate | 0      | UpToDate | true  | *     |
+------------+--------+----------+-------+-------+