Btrfs: don't skip writing out a empty block groups cache

I noticed a slight bug where we will not bother writing out the block group
cache's space cache if it's space tree is empty.  Since it could have a cluster
or pinned extents that need to be written out this is just not a valid test.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
1 file changed