[Tilesathome] .tileset file upload issue fixed

[Sebastian Spaeth]

OK, I fixed one issue with the .tileset file upload for tilesAtHome on
the server.

Empty tilesets as created on the client are denoted with an "emptiness"
value in byte 4 of the .tileset file and the files are much shorter than
a normal valid .tileset file. (it's all spec'd here: http://wiki.openstreetmap.org/wiki/Tiles@home/Tileset_as_one_file_v2)

However the tile serving code could not handle these - in its view -
invalid files (it still expects v1 of the .tileset files) and e.g. the
low zoom stitcher died every time it came across one. So the server now
deletes any existing tileset file if we upload an "empty" one, which
means we fall back to the blankness data provided by "oceantiles.dat".

In the long run we could use that emptiness value to update a server
version of the blankness database.

I feel quite confident that .tileset files upload should work out quite
ok. Dirk, should we throw the switch on the clients? I am ready.

Sebastian