Python - Differentiate `null` from `undefined`

[bmingles]

In dh ui, we currently serialize Python dictionaries dropping None values https://github.com/deephaven/deephaven-plugins/blob/6977a33e61220edc3dbf4a47149b0ce9e96a3b29/plugins/ui/src/deephaven/ui/_internal/utils.py#L105

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