Python - Differentiate null from undefined

[bmingles]

In dh ui, we currently serialize Python dictionaries dropping None values

deephaven-plugins/plugins/ui/src/deephaven/ui/_internal/utils.py

Line 105 in 6977a33

if omit_none and value is None:

This is fine in many cases, but there are others where we need to differentiate null vs undefined values (e.g. selected_key in Picker and ComboBox components).

A way we could address this:

• Create a sentinel value to represent UNDEFINED
• Use the sentinel value as the default for all of our prop signature (e.g. selected_key=UNDEFINED)
• In dict_to_camel_case drop UNDEFINED values but keep None values as null