osd/PrimaryLogPG: encode an empty data_bl for empty sparse reads

osd/PrimaryLogPG: encode an empty data_bl for empty sparse reads

Commit 0cf383da0741 ("ReplicatedPG: clamp SPARSE_READ to object size
for ec pool") didn't handle the case of a sparse read that ends up
being empty correctly: the OSD encodes only an empty extent map whereas
clients (both userspace and kernel) also expect to see an empty data
buffer.  IOW the reply contains one 32-bit zero instead of the expected
two.

Fixes: https://tracker.ceph.com/issues/74394
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>