bindings: Avoid iterating array if it is never set

bindings: Avoid iterating array if it is never set

If query.return_type is not one we want, binding_compose_params() is
not called, and so params remains a NULL pointer. However, the code was
then unconditionally iterating it regardless. Don't if it is still NULL.

CID 1452218 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)
15. var_deref_op: Dereferencing null pointer params.