_filter() can't cope with a weakly-defined parameter

Summary: This shows up in the Cross Stitch Space -- the Completed Projects page is showing an error.

The problem turned out to be fairly subtle, and I'm going to have to think about the right way to handle it in the long run. The underlying expression giving an error was

Project._instances -> _filter(Is Completed)

Is Completed is a complex, multi-level function, which sometimes returns an empty value. In this case, _filter() should almost certainly consider it to be false.

For now, I'm going to code it up that way. But this is probably a long-term question, that can only be fixed by making things more strongly typed.