This handler does nothing. It's intended to be used to avoid the
"No handlers could be found for logger XXX" one-off warning. This is
important for library code, which may contain code to log events. If a user
of the library does not configure logging, the one-off warning might be
produced; to avoid this, the library developer simply needs to instantiate
a NullHandler and add it to the top-level logger of the library module or
package.