ovn-northd: Make it harder to specify a bad database remote.

Without this change, --ovnnb-db='' produces bad results, such as an
assertion failure.  With it, ovn-northd uses the default database.  The
latter seems preferable.  Similarly for --ovnsb-db=''.

Acked-by: Numan Siddique <numans@ovn.org>
Signed-off-by: Ben Pfaff <blp@ovn.org>

northd/ovn-northd.c

@@ -11758,11 +11758,11 @@ parse_options(int argc OVS_UNUSED, char *argv[] OVS_UNUSED)
         }
     }
 
-    if (!ovnsb_db) {
+    if (!ovnsb_db || !ovnsb_db[0]) {
         ovnsb_db = default_sb_db();
     }
 
-    if (!ovnnb_db) {
+    if (!ovnnb_db || !ovnnb_db[0]) {
         ovnnb_db = default_nb_db();
     }