Don't show menu on trigger character when source returns no items

[Saghen]

Feature Description

If the user types a trigger character, we should only show the menu if the source defining that trigger character returns items. We should also define a few more trigger characters that are ignored when entering insert mode and accepting (possible per filetype?) such as {

@folke I'd recommend re-enabling show_on_insert_on_trigger_character after this is closed

[folke]

great!