tst_QStringList: check that join(u'\0') embeds NULs

... as opposed to QByteArray::toHex('\0'), which doesn't.

Picking to all LTSs because it's in a test.

Pick-to: 6.3 6.2 5.15
Change-Id: I392d5714c4865e530a246ed30a13cdac26b98e0b
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/text/qstringlist/tst_qstringlist.cpp

@@ -427,6 +427,11 @@ void tst_QStringList::joinChar_data() const
                         << QLatin1String("c"))
                 << QChar(QLatin1Char(' '))
                 << QString("a b c");
+
+    QTest::newRow("null separator")
+            << QStringList{QStringLiteral("a"), QStringLiteral("b"), QStringLiteral("c")}
+            << QChar(u'\0')
+            << QStringLiteral("a\0b\0c");
 }
 
 void tst_QStringList::joinEmptiness() const