support null/is_null sub-ops inside has/any ops

The code in `_sub_operator` was assuming that the sub-operator dictionary argument had a `val` key, but in the case of the `null` and `is_null` operators, there is no `val` key, so we use `get` and pass `None` to `_create_operation`.
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Merge pull request #325 from goodscloud/has-any-null-ops

support null/is_null sub-ops inside has/any ops