The assumption when calling QStringRef::toString() on a null
QStringRef (i.e. when QStringRef::isNull() is true) is that
QStringRef::toString().isNull() will also return true. With the
current implementation we return a null QString() only when the
QStringRef references a nullptr QString. We need to do the same
also when QStringRef references a QString with null private data.
Change-Id: I4177a5ea187ae758d7c46fe76a9d0583140e90cb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
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