fix(vim.ui.input): return empty string when inputs nothing
The previous behavior of `vim.ui.input()` when typing <CR> with
no text input (with an intention of having the empty string as input)
was to execute `on_confirm(nil)`, conflicting with its documentation.
Inputting an empty string should now correctly execute `on_confirm('')`.
This should be clearly distinguished from cancelling or aborting the
input UI, in which case `on_confirm(nil)` is executed as before.
59ff4691f6
