doc/_ext: ignore desc if it is unavailable

there is chance that we don't have desc, desc_long or fmt_desc, in that
case, we should just skip desc before checking its length. so, just use
'if desc' which is able to check for None or empty string.

Signed-off-by: Kefu Chai <kchai@redhat.com>

doc/_ext/ceph_confval.py

@@ -19,7 +19,7 @@ logger = logging.getLogger(__name__)
 
 TEMPLATE = '''
 .. confval_option:: {{ opt.name }}
-{% if desc | length > 1 %}
+{% if desc %}
    {{ desc | wordwrap(70) | indent(3) }}
 {% endif %}
    :type: ``{{opt.type}}``