PipeOpFFS: Address edge cases with no evaluations for an individual

[sebffischer]

mlr3fda/R/PipeOpFFS.R

Line 239 in 14b8da8

ffind = function(x, left = -Inf, right = Inf) {

assumes that x has length > 0

[sebffischer]

More links:

• unexpected warning with only one evaluation tidyfun/tf#34
• empty edge case errs tidyfun/tf#33
• https://github.com/slds-lmu/mlr3reports/tree/main/MissingValueTFD

[sebffischer]

If tf starts supporting this, we need to add it is a feature. Otherwise we should add a test that the creation of such data with tf fails indeed (so at least we are reminded when we have to support it as well). If this is not the case by end of march it needs to be properly documented somewhere.