The NullHandler class in git.util was added when merging gitpython-developers#300, to
allow a noop handler to be used on Python 2.6, since the standard
library logging.NullHandler class was added in Python 2.7.

When introduced in d1a9a23, the git.util.NullHandler class was also
patched into the logging module, but that has no longer been done
since 2fced2e (gitpython-developers#979), nor does GitPython make other use of it.

This also changes the parameter type annotation on the emit method
from `object` to `logging.LogRecord`, which is the expeced type.