TileHandlerEmpty: Share the buffer of zeros

Share the same buffer of 0s between all empty handler instances.
tests: Add test case for shared empty tile
Remove "babl-tolerance" from GeglConfig

Because of the way babl parses this variable from the environment
the config value never really worked as expected. Since it is
causing some unrelated issues with printf and locale I am electing
to just drop the setting. It can still be modified by setting the
environment variable before calling init if you really know what
you're doing.
oilify: GCC warning about uninitialized value

The uninitialized path was not reachable, but knowing
that requires assumptions the compiler can't make.