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 aec367ef28)

src/tools/qstring.cpp

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