Changes

- remove the #errorEmptyCollection in #atRandom on Collection, as we do
an emptyCheck in the first line - simplify Bag>>#atRandom: as described
in the issue tracker entry
We could simplidy Collection>>#atRadom more if we would have
withIndexDo: on Collection (and I really wonder why we do not have that)
fixes #5631

- remove the #errorEmptyCollection in #atRandom on Collection, as we do
an emptyCheck in the first line - simplify Bag>>#atRandom: as described
in the issue tracker entry
We could simplidy Collection>>#atRadom more if we would have
withIndexDo: on Collection (and I really wonder why we do not have that)
fixes #5631