Modernize NULL.

[1uc]

With the introduction of generic_data_handle a NULL "pointer" will stop behaving as if it where a pointer, in certain circumstances (Context: in C++ it's defined as 0 not (void*) 0 as in C). This means that occasionally,

_pp = NULL;
_pp = nullptr;

could be two very different things.

The task would be the very mechanical process of replacing every NULL with nullptr.

[pramodk]

Just to mention from Slack discussion: https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-nullptr.html