Join optimizer: fanout of LEFT JOIN cannot be less than one

Filing this based on MDEV-34097.

When the join optimizer produces a query plan for a construct like

t1 LEFT JOIN (t2 JOIN t3 ON ...) ON ...

it picks a join order of

t1 t2 t3 

Suppose the fanout of the t2, t3 part is less than 1.

The fanout of t1, t2, t3 cannot be less than fanout of t1 , because LEFT JOIN will always generate at least one matching record combination for each record of t1.