UB when passing null string to QString::arg()

UBsan complains when building qtdeclarative because a null pointer is passed as an argument to memcpy when that argument is declared non-null.

This happens because a null string is passed to QString::arg().

Passing a null string to QString::arg() may seem silly but it can occur in code where it would make sense to expect it to work.
https://codereview.qt-project.org/c/qt/qtdeclarative/+/528756?tab=comments

The documentation for QString::arg() doesn't say anything about not expecting a null string.