Another cross-cutting, large-scale clang-tidy fix. It runs the
“modernize-use-nullptr” fixit over all source code compiled in a fairly
full-featured MySQL build, changing 0 literals (in pointer context)
and NULL to nullptr. A typical example would be:

-  mysql_options(connection, MYSQL_OPT_CONNECT_ATTR_RESET, 0);
+  mysql_options(connection, MYSQL_OPT_CONNECT_ATTR_RESET, nullptr);

as the third argument to mysql_options() is a void *.

Converting 0 and NULL literals to nullptr makes code easier to read by
increasing self-documentation, makes type and argument order mistakes more
difficult, enable compiler warnings when nonsensical operations are attempted
(such as adding pointers), enable perfect forwarding in more cases (NULL cannot
be perfect forwarded, nullptr can) and reduces noise from IDEs and other tools
that check clang-tidy warnings.

Change-Id: Ic863d0a52a78b46f6345e3b69b60bd3dab317e9a