scripts: menuconfig: proper handling of NULL character as input

Fixes: zephyrproject-rtos/zephyr#33212

Ignoring when user inputs NULL in a text field.
menuconfig exits with a python stack trace if NULL is provided as input
character, therefore ignore NULL as an input character to prevent this
behaviour.

A NULL character may be given accidentally by the user through the
following ways:
- Pressing `Win` key on keyboard (Windows only)
- Pressing `<CTRL>-@` / `<CTRL>-2`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

Author: tejlmand

menuconfig.py

@@ -1757,6 +1757,9 @@ def edit_width():
             _safe_curs_set(0)
             return None
 
+        elif c == "\0":  # \0 = NUL, ignore
+            pass
+
         else:
             s, i, hscroll = _edit_text(c, s, i, hscroll, edit_width())
 
@@ -2196,6 +2199,9 @@ def select_prev_match():
         elif c == curses.KEY_HOME:
             sel_node_i = scroll = 0
 
+        elif c == "\0":  # \0 = NUL, ignore
+            pass
+
         else:
             s, s_i, hscroll = _edit_text(c, s, s_i, hscroll,
                                          _width(edit_box) - 2)