QByteArrayList: micro-optimize join(QByteArray)

Null- vs. emptiness of the separator is not significant for join(), so
skip the isNull() check in the conversion ctor of QByteArrayView from
QByteArray by using the named conversion function that exists for this
purpose instead.

Pick-to: 6.3
Change-Id: I6ef07cc9bcc0bc8b87ecadc5cfaac9793cfb1b77
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/text/qbytearraylist.h

@@ -86,7 +86,7 @@ public:
     }
     Q_WEAK_OVERLOAD
     inline QByteArray join(const QByteArray &sep) const
-    { return join(QByteArrayView{sep}); }
+    { return join(qToByteArrayViewIgnoringNull(sep)); }
     inline QByteArray join(char sep) const
     { return join({&sep, 1}); }
 };