Automatically enable "clear_button" selectize plugin when `choices` include `""`

[gadenbuie]

In the app below, choices includes a "" value. This makes it possible for the user to select "nothing". The problem is that, from a usability perspective, it's hard to know how to enter the "nothing" state.

You can try this on shinylive:

1. Select a choice from the input.
2. Without using the clear button, attempt to clear the current selection.
3. The correct actions are:
   1. Move focus to the selectize input
   2. Press Backspace to delete the current input
   3. Do not press Enter, instead Tab or move focus away from the input.

We could instead detect the presence of "" in choices and turn on the "clear_button" plugin by adding it to the options["plugins"] array.

from shiny.express import input, render, ui


ui.input_selectize(
    "filter",
    "Filter by",
    choices={
        "": "Select filter",
        "first": "First Choice",
        "second": "Second Choice",
    },
    options={
        "plugins": ['clear_button']
    }
)


@render.text
def txt():
    if input.filter():
        return f"Data is filtered by your {input.filter()} choice."
    else:
        return "The data is not filtered."

[cpsievert]

In the app below, choices includes a "" value. This makes it possible for the user to select "nothing".

It's actually possible to still select nothing with a "" choice. For example, hit backspace on the app below:

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

ui.input_selectize(
    "foo",
    "Foo",
    choices=["a", "b"],
    multiple=False,
)


@reactive.effect
def _():
    print(f"Selected value: {input.foo()}")

and you'll see:

Selected value: a
Selected value: 

In light of this, it seems like a good idea to add "clear_button" across-the-board since it makes it more obvious that "" is a totally valid state that you should account for.