QDoc: Minor cleanup in generator.cpp

Fix 0 as nullptr constant.
Stop using C-style casts:
- Do not cast to int that which returns an int (QString::length())
- Use static_cast<>() where it's sensible to do so.

Change-Id: I48c7a3e6f1465759dd3973eff66656a4f2a1c19a
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>