Minor, make Phriction migration a little more robust

On secure.phabricator.com, we have at least one page with a bad project slug
from long ago. Make the migration recover in this case.

Auditors: btrahan
Make Phriction policy migration even more robust

We have at least one project with `null` as a viewPolicy. This should get
sorted out separately, but make the migration robust against it.

Auditors: btrahan