moc: use Q_NULLPTR instead of 0 in generated code.

This commit changes several instances where moc was generating code that
used 0 as a null pointer constant. The Q_NULLPTR define is the more
idiomatic way to do this, and additionally this silences warnings
generated by e.g. GCC's -Wzero-as-null-pointer-constant.

[ChangeLog][Tools][moc] Fixed "zero as null pointer constant" warnings
in moc's generated code.

Change-Id: Ibe382b7bdbdddaf20cb4bdfd075fcdd1f363f9d3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>