MXNET-1444/MXNET-1445: replace NULL by nullptr literal

[olk]

• using nullptr is more explicit
• macro NULL can be a integer literal with value zero (since C++11)
• literal nullptr is of type std::nullptr_t
• as template arg NULL and 0 evaluate to integer

[leezu]

Please see CI, lint is failing:

[2020-02-06T19:25:21.265Z] include/mxnet/lib_api.h: 1 Errors of 1 Categories map={'whitespace': 1}

[olk]

whitespace error was introduced by another commit

ae472c2 dynamic custom operator support (#15921)

I've fixed it

[leezu]

[2020-02-06T20:48:22.104Z] include/mxnet/lib_api.h:631: Lines should be <= 100 characters long [whitespace/line_length] [2]

Sorry, I didn't paste the exact error message but only the error summary before.

[olk]

There was an additional whitespace error from a previous commit - I don't know why it could pass the lint check.
I've adopted my vimrc accordingly.

Could I get a link to the coding/code formatting standards used by MXNet?

[olk]

generating Julia docs fails with:

[2020-02-07T10:19:19.325Z] raise VersionConflict(dist, req).with_context(dependent_req)
[2020-02-07T10:19:19.325Z] pkg_resources.VersionConflict: (Markdown 3.2 (/home/jenkins_slave/.local/lib/python3.5/site-packages), Requirement.parse('markdown<3.2'))

Seams to be an problem of the build system.