Don't assign pipeop predict type if it does not change anything

[mb706]

... since PipeOp might be upset about assignments, even if they don't do anything

mlr3pipelines/R/GraphLearner.R

Line 369 in cb8265b

walk(predict_type_pipeops, function(po) po$predict_type = rhs)

at the same time, we want a !identical check here

mlr3pipelines/R/PipeOp.R

Line 406 in cb8265b

if (!missing(val)) {

(we still want both because some 3rd party PO might have read-only active binding