Fix compilation for Linux

[yaynstuff]

Compilation on linux fails with:

dullahan_host.cpp:33:51: error: conversion from ‘long int’ to ‘CefRefPtr’ {aka ‘scoped_refptr’} is ambiguous
33 | return CefExecuteProcess(main_args, NULL, NULL);
| ^

Changing NULL to nullptr allows compilation to proceed.

[callumprentice]

Looks good to me. Thank you.

[bennettgoble]

@callumprentice is it okay to merge these changes into main or do we need to wait until the changes are integrated into a viewer that is promoted to default release?

[nat-goodspeed]

Callum and I are discussing, and would like to surface this question at our upcoming meeting.