Fix handling of nil values in PII filtering

[iangreenleaf]

I've had a couple errors come into Squash that it has failed halfway through creating (leaving my DB in a bad state). In tracing them back, what I think happened is that ivars came in with a hash containing nil values, which causes the filtering method to fail.

I've fixed it here by simply recursing through hash structures, which seems like an elegant and safe way to make it happen.