Add support for `AutocompleteInput(min_characters=0)`

[mattpap]

https://user-images.githubusercontent.com/27475/183263668-d4b177ff-4e70-4fcf-a14c-b54b0e1ba434.mp4

fixes #12272

[bryevdv]

@mattpap what's left for this? If you can describe what's remaining, I can try to push this to finish so you can focus on layout.

[mattpap]

I thought there were integration tests to fix, but it looks like those were spurious failures. I intended to add bokehjs integration tests for this and perhaps migrate existing non-server integration tests to bokehjs, but that can be done in another PR.

[bryevdv]

Let me see if I can add some tests today/tomorrow