Can't get data for empty compressed resources

If you have a resource file that is empty (for example /org/gnome/gitg/ui/style-unix.css in gitg) which is marked for compression, then it expands from 0 to 16 bytes, but the target size is still recorded (0 bytes). This is then passed into g_converter_convert() with the zlib decompressor.

However, this then runs into this assert in g_converter_convert():

	  g_return_val_if_fail (outbuf_size > 0, G_CONVERTER_ERROR);

Which fails the entire op.

I'm not sure what parts are wrong or not here. But here are some ideas:

1. Don't auto-compress zero sized (or generally any files where the compressed data ends up larger).
2. Skip decompression for zero sized targets
3. Pass an extra byte for target buffer size
4. Make the assert be >= 0 and make sure all decoders properly handle target size checks that are 0 bytes (which might be hard).