wsrep_node_is_ready() check is too strict

The check that prints the dreaded "WSREP has not yet prepared node for application use" is sometimes too strict. It doesn't allow any selects, even if these selects do not access any tables. For example, I'd think that

SELECT 1;

or, for example,

SELECT @@max_allowed_packet;

could be allowed. This is easy to do, one should not report this error if all_tables is NULL.