kconfig: qconf: clean deprecated warnings

The recommended way to initialize a null string is with
QString(). This is there at least since Qt5.5, with is
when qconf was ported to Qt5.

Fix those warnings:

	scripts/kconfig/qconf.cc: In member function ‘void ConfigItem::updateMenu()’:
	scripts/kconfig/qconf.cc:158:31: warning: ‘QString::null’ is deprecated: use QString() [-Wdeprecated-declarations]
	  158 |    setText(noColIdx, QString::null);
	      |                               ^~~~
	In file included from /usr/include/qt5/QtCore/qobject.h:47,
	                 from /usr/include/qt5/QtWidgets/qwidget.h:45,
	                 from /usr/include/qt5/QtWidgets/qmainwindow.h:44,
	                 from /usr/include/qt5/QtWidgets/QMainWindow:1,
	                 from scripts/kconfig/qconf.cc:9:
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/qconf.cc

@@ -154,9 +154,9 @@ void ConfigItem::updateMenu(void)
 
 		if (!sym_is_changeable(sym) && list->optMode == normalOpt) {
 			setPixmap(promptColIdx, QIcon());
-			setText(noColIdx, QString::null);
-			setText(modColIdx, QString::null);
-			setText(yesColIdx, QString::null);
+			setText(noColIdx, QString());
+			setText(modColIdx, QString());
+			setText(yesColIdx, QString());
 			break;
 		}
 		expr = sym_get_tristate_value(sym);
@@ -276,7 +276,7 @@ void ConfigLineEdit::show(ConfigItem* i)
 	if (sym_get_string_value(item->menu->sym))
 		setText(QString::fromLocal8Bit(sym_get_string_value(item->menu->sym)));
 	else
-		setText(QString::null);
+		setText(QString());
 	Parent::show();
 	setFocus();
 }