bug(interactive-repr): interactive repr of null date scalar raises an exception

[cpcloud]

What happened?

In [10]: from ibis.interactive import *

In [11]: s = ibis.null("date")

In [12]: s

raises an exception:

│ …/ibis/expr/types/pretty.py:139 in _                                                             │
│                                                                                                  │
│   136 @format_values.register(dt.Date)                                                           │
│   137 def _(dtype, values, **fmt_kwargs):                                                        │
│   138 │   dates = [v.date() if isinstance(v, datetime.datetime) else v for v in values]          │
│ ❱ 139 │   return [Text.styled(d.isoformat(), "magenta") for d in dates]                          │
│   140                                                                                            │
│   141                                                                                            │
│   142 @format_values.register(dt.Time)                                                           │
╰──────────────────────────────────────────────────────────────────────────────────────────────────╯
AttributeError: 'NoneType' object has no attribute 'isoformat'

What version of ibis are you using?

main @ 12a45cf

What backend(s) are you using, if any?

No response

Relevant log output

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