change the inhibition value of *_SAVE_AS to ''

Previously, the documentation claimed the value of None for this purpose
even though False was used for certain defaults. The values False and
None cause warnings to be emitted from URLWrapper._from_settings though,
so the new way of inhibiting page generation is to set a *_SAVE_AS value
to the empty string.

Note: I am submitting this on the behalf of helmut who can be found on IRC with that name.