feat: Make MISSING_TYPE a None-alike object

[gadenbuie]

This PR updates MISSING_TYPE to make it behave similar to a None object by implementing several dunder methods. It also updates reactive.Value() to return the MISSING value when accessing an uninitialized reactive, rather than raising a SilentException.

This PR is current a proof-of-concept: there are a lot of red squiggles to track down where previously we relied on the SilentException to prevent a non-initialized input value from propagating. I'll continue to investigate and resolve these as I have time.

Behavior

>>> import shiny
>>> missing = shiny.types.MISSING
>>> missing
<MISSING>
>>> str(missing)
''
>>> missing or "default"
'default'
>>> 'yes' if missing else 'no'
'no'
>>> # No two missing values are ever the same
>>> missing == missing
False
>>> isinstance(missing, shiny.types.MISSING_TYPE)

Example App

Before this PR, the following app would never render the slider because calling input.slider() would raise an exception and the ui.input_slider() would never be returned.

from shiny import render
from shiny.express import input, ui

ui.input_switch("show_slider", "Show slider", True)


@render.ui
def uiElement():
    if input.show_slider():
        value = input.slider() or 5
        return ui.input_slider("slider", "Choose a number", min=1, max=10, value=value)

This could be worked around by calling

value = input.slider() if "slider" in input else 5

but that's not as concise or pythonic as value = input.slider() or 5.

[schloerke]

I'd like to throw an option into the ring that an uninitialized reactive.Value() throws an error and req() can catch the error.

Having to handle MISSING_TYPE for every reactive.Value() would be cumbersome to handle.

[gadenbuie]

@schloerke Agreed, that makes a lot of sense