Use empty d->cString for TQString::shared_null instead of null string.

This restores consistency for the behavior of methods TQString::utf8()
and TQString::local8Bit() with null TQStrings.

Signed-off-by: Slávek Banko <slavek.banko@axis.cz>
(cherry picked from commit e99bbda734)

src/tools/qstring.cpp

@@ -5992,7 +5992,7 @@ void QString::setSecurityUnPaged(bool lock) {
 QCString QString::utf8() const
 {
     if (!d->cString) {
-        d->cString = new QCString;
+        d->cString = new QCString("");
     }
     if(d == shared_null)
     {
@@ -6246,7 +6246,7 @@ QString QString::fromLatin1( const char* chars, int len )
 QCString QString::local8Bit() const
 {
     if (!d->cString) {
-        d->cString = new QCString;
+        d->cString = new QCString("");
     }
     if(d == shared_null)
     {