Fix autocompletion bar not hiding when escaping empty command bar

[SaadZahem]

Fixes #1035

Root cause: in girara-gtk/completion.c, girara_isc_completion() starts with:

if (input_length == 0 || input[0] != ':') {
    return false;
}

This runs before the code that checks argument->n == GIRARA_HIDE and actually destroys/hides session->gtk.results. girara_isc_abort() (which Escape triggers via sc_abort → girara_sc_abort) calls girara_isc_completion() with GIRARA_HIDE to close the popup — but once you've backspaced the input bar down to nothing (even the leading : is gone), input_length == 0 and the function bails out immediately, so the hide logic never runs and the completion pane stays on screen.

Fix: let a GIRARA_HIDE request through regardless of the current input bar contents:

if ((input_length == 0 || input[0] != ':') && argument->n != GIRARA_HIDE) {
    return false;
}

I took time to build (zathura + zathura-pdf-mupdf) and test this on my arch linux machine.
Let me know if anything needs to be edited before a merge.