net/mlx5: fw_tracer, allow 0 size string DBs

Device can expose string DB of size 0 which means this string DB is
currently not in use. Therefore, allow for 0 size string DBs.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>