QLocale/Win: don't move from an lvalue

The nullIfEmpty() function took its argument by universal reference,
but then unconditionally moved from it into a QVariant. As long as
QVariant didn't have ctors from rvalue-refs, this wasn't an actual
move, but still a copy (std::move() doesn't move, it's an
lvalue-to-rvalue (xvalue) cast). With the implementation of move-ctors
for QVariant (QTBUG-53265), this suddenly becomes a move and, in
zeroDigit(), moves away from the 'zero' member.

Issue location found by Copilot.

Fix by forwarding from the universal reference instead of
moving. Contrary to what Copilot keeps telling me, this _is_ the
correct fix.

Amends 385626d.

Pick-to: 6.10 6.8 6.5
Change-Id: I91cba49f39506d403b3f7a21c9f4b58fd91af8d5
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit ad0b53f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: marcmutz

src/corelib/text/qlocale_win.cpp

@@ -99,7 +99,7 @@ static QVariant nullIfEmpty(T &&value)
     // For use where we should fall back to CLDR if we got an empty value.
     if (value.isEmpty())
         return {};
-    return std::move(value);
+    return std::forward<T>(value);
 }
 }