R1:07082d28678c

Don't allow empty list constraints in Conduit calls

Summary:
Ref T11473. If you write a method like `get_stuff(ids)` and then call it with an empty list of IDs, you can end up passing an empty constraint to Conduit.

If you run a `*.search` method with such a constraint, like this one:

```
{
"ids": []
}
```

...we have three possible beahviors:

# Treat it like the user passed no constraint (basically, ignore the constraint).
# Respect the constraint (return no results).
# Error.

Currently, we…