I was using both `yaml` and `json2huddle` at the same time, and the JSON input had a `null` value.

the error can be more simply reproduced by

e.g.
```
  tclsh
  % package require yaml
  0.4.1
  % package require huddle::json
  0.1
  % package require huddle
  0.4
  % ::huddle null
  Invalid subcommand 'null' for type '::huddle::Type_null'
```

IMO - no expert on this:

I think this is because `yaml` and `huddle` clash in the `ensemble` null command.  Also, `yaml` appears to map this to a `string` when huddle expects `null` as it's own type.

I tested a small patch (attached) - it seems to pass existing tests, and tweaking the input to any of the test files to mirror the pkgIndex.tcl does seem to show this failure without.