tst_QSplitter: fix compiler warning

QLabel *l is declared uninitialized, assigned in a for loop. The last
object is deleted for testing purposes.
This leads to a false compiler warning about deleting a potentially
unintialized pointer.

=> initialize with nullptr to silence the warning.

Pick-to: 6.5 6.6
Change-Id: I1422b04fc1fdbfc7248de577884aabfb539f3f4b
Reviewed-by: Doris Verria <doris.verria@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

tests/auto/widgets/widgets/qsplitter/tst_qsplitter.cpp

@@ -1032,7 +1032,7 @@ void tst_QSplitter::taskQTBUG_4101_ensureOneNonCollapsedWidget()
     QFETCH(bool, testingHide);
 
     MyFriendlySplitter s;
-    QLabel *l;
+    QLabel *l = nullptr;
     for (int i = 0; i < 5; ++i) {
         l = new QLabel(QString("Label ") + QChar('A' + i));
         l->setAlignment(Qt::AlignCenter);