[Clang] Prevent null pointer dereference in Sema::​CodeCompleteQualifiedId() (#90490)

The null pointer dereference issue seems happening with in the
expression NNS->getAsType().

Although dyn_cast_or_null<TemplateTypeParmType>() correctly handles null
pointers, it doesn’t prevent the subsequent dereferencing operation.

The fix ensures that NNS pointer is not null before calling the
getAsType() method, thus preventing potential runtime errors caused by
attempting to access a null pointer.