I'm using `traitlets` version 5.14.3 and Python 3.12.3... In both cases below, a `TraitError` is not thrown, even though `None` is assigned to a `Unicode()` field after initialization... ```python from traitlets import HasTraits, Dict, Unicode class Foo(HasTraits): data = Dict(per_key_traits={"a_key": Unicode()}) class Bar(HasTraits): data = Dict(key_trait=Unicode(), value_trait=Unicode()) foo = Foo() foo.data = {"a_key": "a_value"} # I think this should throw a TraitError... foo.data["a_key"] = None bar = Bar() bar.data = {"a_key": "a_value"} # I think this should throw a TraitError... bar.data["a_key"] = None ```