mds: display sane hex value (0x0) for empty feature bit

mds: display sane hex value (0x0) for empty feature bit

Print a valid hex (0x0) during empty feature bit, so that the clients could recognize it.
When the _vec size becomes 0, print() function creates an invalid hex (0x) and 'perf stats'
crashes with the below error:
"
File "/opt/ceph/src/pybind/mgr/stats/fs/perf_stats.py", line 177, in notify_cmd
metric_features = int(metadata[CLIENT_METADATA_KEY]["metric_spec"]["metric_flags"]["feature_bits"], 16)
ValueError: invalid literal for int() with base 16: '0x'
"
This patch creates  a valid hex (0x0), when _vec size is 0.

Fixes: https://tracker.ceph.com/issues/59551
Signed-off-by: Jos Collin <jcollin@redhat.com>