Set DEBUG and TESTING config value to default to None, not False.

Rationale: Some code might want to distinguish between "DEBUG has not been
set, determine correct value from the environment / other means" and "DEBUG
has been turned off explicitly".

One user of this is the Flask-Script extension, which would like to use a
default of True for the DEBUG setting when its built-in script runner is
used. Rationale: Many people use a special gunicorn-or-whatever runner in
production, but start a quick local "manage.py runscript" when they want to
test something / reproduce a bug. In this case it's helpful to not require
a different set of options that the user has to remember, or to have to use
a second config file which could go out of date.