KeysPrefix: do not transform empty prefix

When KeysPrefix was introduced, the behaviour of Keys was changed:
rather than just walking from BasePath, it attempted to transform the
empty prefix using the configured BlockTransform. I do not think this is
correct behaviour; it just happened to be fine in the default case
because the default transform used in tests would simply return an empty
slice, which means pathFor() would end up returning BasePath anyway.
However, BlockTransform functions should arguably not be expected to
successfully transform an empty key into an empty slice (ours actually
panics in that scenario which is how I noticed this ;-)
Merge pull request #20 from jonboulle/panic

KeysPrefix: do not transform empty prefix