If we have an input parameter (or return value) we need to use (nullable).
However, if it is an (inout) or (out) parameter, (optional) is sufficient.
It looks like (nullable) could be used for everything according to the
Annotation documentation, but (optional) is more specific.
Add GZlibCompressor:file-info property. If it contains a non-NULL
GFileInfo, and the compressor is in GZIP mode, the filename and
modification time from the file info are written to the GZIP header
in the output data.
Add GZlibDeompressor:file-info property. If the decompressor is in GZIP
mode, and the GZIP data contains a GZIP header, the filename and
modification time are read from it, stored in a GFileInfo, and the
file-info property is notified.
Bug #617691.
