Ignore nulls from terminal CTRL-space in LineEdit

By default some terminals (eg, xterm) generate '\0' when CTRL-space is
pressed.  However, '\0' is used internally by the key map system to
represent a wildcard (see normalize_key()), so cannot be handled without
a larger overhaul.  Instead, just ignore it to avoid user confusion with
lines containing an invisible, unparsable character.

Fixes JuliaLang#7001

base/LineEdit.jl

@@ -725,6 +725,9 @@ function match_input(k::Dict, s, term=terminal(s), cs=Char[], keymap = k)
  # return an empty keymap function
  eof(term) && return keymap_fcn(nothing, "")
  c = read(term, Char)
+ # Ignore any '\0' (eg, CTRL-space in xterm), as this is used as a
+ # placeholder for the wildcard (see normalize_key("*"))
+ c != '\0' || return keymap_fcn(nothing, "")
  push!(cs, c)
  key = haskey(k, c) ? c : '\0'
  # if we don't match on the key, look for a default action then fallback on 'nothing' to ignore