kconfig: qconf: use preferred form of QString API

kconfig: qconf: use preferred form of QString API

A QString constructed from a character literal of length 0, i.e. "", is not
"null" for historical reasons. This does not matter here so use the preferred
method isEmpty() instead.

Also directly construct empty QString objects instead of passing in an empty
character literal that has to be parsed into an empty object first.

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Link: https://doc.qt.io/qt-6/qstring.html#distinction-between-null-and-empty-strings
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>