* Return empty colors for .cxg v0.0 files
Fixes https://github.com/chanzuckerberg/cellxgene/issues/1440
The CxgAdaptor.get_colors method currently assumes that the .cxg file has
cxg_group_metadata. As a result, the /api/v0.2/colors endpoint always fails for
.cxg v0.0 files.
* Add test fixture
